Allogeneic hematopoietic cell transplantation (HCT) represents a potentially curative treatment modality in a range of hematologic malignancies. High-dose myeloablative radio-chemotherapy has conventionally been used as part of the preparative regimen before HCT for two reasons: it has a profound immunosuppressive effect on the host, limiting the ability to reject the graft and it has substantial anti-tumor efficacy. Graft rejection is an example of alloreactivity as alloreactivity denotes the immunologic reactions that occur when tissues are transplanted between two individuals within the same species. If the immune system of the host is suppressed to a degree where rejection does not occur, the possibility arises that immunocompetent donor cells can attack the recipient tissues. This phenomenon is termed Graft-versus-Host disease (GVHD) if healthy tissues of the host are attacked and the Graft-versus-Tumor (GVT) effect if the malignant cells are the targets of the reaction. Clinical studies have shown that patients who develop GVHD have a lower risk of relapse of the malignant disease and that donor lymphocyte infusion can induce durable remissions in patients with relapsed disease following the transplant. These observations indicate that a GVT effect can be present following allogeneic HCT and that this effect, like GVHD, is an alloreactive response. The toxicity of HCT with myeloablative conditioning is considerable and this limits the use of this procedure to patients below 50-60 years of age. A large proportion of the patients with hematologic malignancies are older than 60 years at diagnosis and they are therefore not eligible for this treatment. During the last decade, conditioning regimens that are nonmyeloablative or have reduced intensity have been developed. The purpose of this development has been to extend the use of allogeneic HCT to older patients and to patients who due to the malignant disease or to comorbidities are unable to tolerate myeloablative conditioning. In allogeneic HCT with nonmyeloablative conditioning the curative potential relies entirely on the ability of the donor cells to elicit a GVT effect. Allogeneic HCT with nonmyeloablative conditioning was introduced at Department of Hematology at Rigshospitalet in March 2000. The results of this treatment modality have been promising and we and others have shown that durable remissions can be obtained in patients who are heavily pretreated. One of the goals of allogeneic HCT with nonmyeloablative conditioning was to perform both the actual transplant procedure and the clinical follow up in the outpatient setting. In the first 30 patients transplanted at Rigshospitalet, we observed that the transplant itself and the first weeks post transplant could be performed as an outpatient procedure in a number of patients. However, all the patients were admitted and the median duration of hospitalization was 44 days during the first year post transplant. Complications such as infections and GVHD were common causes of hospitalization and studies from other centers have shown that infections, GVHD and relapse of the malignancy are the major obstacles to a good result of allogeneic HCT with nonmyeloablative conditioning. One way to improve the results of this treatment would therefore be to reduce the incidence of GVHD without compromising the GVT effect. In HCT with nonmyeloablative conditioning the relatively well-defined antineoplastic effect of high-dose myeloablative radio-chemotherapy is substituted with the alloreactive effect of the donor cells. Because the level of alloreactivity varies widely between different donor-recipient pairs, the ability to monitor the level of alloreactivity following the transplant would therefore be desirable. To this end we have investigated the ability of different immunologic and molecular methods to quantify the level of ongoing alloreactivity following the transplant. By simultaneous determination of the fraction of T cells of donor origin (donor T-cell chimerism) and the total number of T cells in the peripheral blood, we observed that patients with a high number of donor CD8 + T cells on day +14 had a high risk of acute GVHD. Other studies have shown that the level of donor T-cell chimerism early after transplant predicts the development of acute GVHD. One way to exploit this knowledge could be to individualize the pharmacologic immunosuppression given post transplant. This immunosuppression is given primarily to prevent the development of GVHD but may also inhibit the GVT effect. In patients with a low risk of GVHD early tapering of the immunosuppression could be done, while the period of immunosuppression could be extended in patients with a high risk of GVHD. In this way the GVT effect could theoretically be optimized in each patient and the results of the treatment improved. In another study we used limiting dilution analysis to monitor the frequencies of interleukin (IL)-2 producing helper T cells responding to recipient or donor antigens following the transplant. The conclusion from this study was that both the technical performance and the data analysis were to complex for this method to be used as a routine clinical tool. However, the study showed that immune responses following HCT are subject to a tight regulation and suggested that this regulation could be due to regulatory cell populations. Such regulatory cell populations have been used successfully in animal models to treat acute GVHD. The secretion of cytokines is an important aspect of immune responses. We analyzed cytokine gene expression in mononuclear cells obtained from patients and donors before and after HCT. Patients with acute GVHD had lower levels of IL-10 mRNA on day +14 than patients who did not develop acute GVHD. Patients who experienced progression or relapse of the malignant disease were characterized by higher levels of IL-10 mRNA before the transplant than patients who remained in remission. The conclusion of this study was that IL-10 might be an inhibitor of alloreactivity following allogeneic HCT with nonmyeloablative conditioning. Allogeneic HCT with nonmyeloablative conditioning represents a major step forward in the treatment of patients with hematologic malignancies. However, many issues such as whom to transplant and when the transplant should be performed remain to be clarified. Large prospective studies, involving collaboration between centers, are needed to define the role of HCT with nonmyeloablative conditioning along with other treatment modalities. In addition, it is important to continue to elucidate the immunologic mechanisms that are responsible for GVHD and the GVT effect.