Accompanying comorbidities observed during the cancer treatment usually affect the course and outcome of the therapy. Hypovitaminosis D, which is one of these conditions, is a resolvable problem, if recognized. In this study, we investigated whether the serum 25(OH)D levels of the patients who were presented to our outpatient clinic were different from the serum levels of the healthy population living in the same area. Our study included 90 patients who were presented to the Medical Oncology outpatient clinic and 90 age, gender, body mass index and ethnic origin matched controls without a known disease, who were presented to the outpatient clinics of the Departments of Internal Diseases and Family Medicine for routine controls. Blood count tests, detailed biochemistry tests (including serum levels of Cr, Ca and P), measurement of serum 25(OH)D levels and C-reactive protein were performed in serum samples of all of the patients and controls. Mean serum levels of 25(OH)D were 13.5 ng/ml (SD 5.1) in all cancer patients, 13.1 ng/ml (SD 4.2) in the patients who were presented for adjuvant therapy, 13.8 ng/ml (SD 5.5) in the patients who were presented at metastatic stage and 18.4 ng/ml (SD 12.5) in the controls. Mean serum CRP levels were 5.4 mg/dl (SD 1.2) in the control group, 8.4 mg/dl (SD 4.3) in the adjuvant therapy group and 20.3 (SD 16.8) in the patients with metastatic disease. Generally, all cancer patients (p 0.003) and the patients with metastatic cancer (p 0.004) had lower serum 25(OH)D levels compared to controls, and there was an inverse correlation between serum 25(OH)D and CRP levels in patients with metastatic cancer (p 0.036). In metastatic cancer patients, hypovitaminosis D may be a comorbidity and it is recommended to consider during initial evaluation and follow-up. Because it might improve these patients quality of life and chemotherapy adherence.