Six evident lesional steps of tumor progression form the neoplastic system that affects the human epidermal melanocyte: 1) the common acquired melanocytic nevus; 2) a melanocytic nevus with lentiginous melanocytic hyperplasia, i.e., aberrant differentiation; 3) a melanocytic nevus with aberrant differentiation and melanocytic nuclear atypia, i.e., melanocytic dysplasia; 4) the radial growth phase of primary melanoma; 5) the vertical growth phase of primary melanoma; and 6) metastatic melanoma. The common acquired melanocytic nevus is viewed as a focal proliferation of melanocytes, destined in most instances to follow a programmed pathway of differentiation that leads to disappearance of the nevus. If the pathway of differentiation is not followed, characteristic lesions result, and such lesions are regarded as the formal histogenetic precursors of melanoma. Such a developmental flaw is termed aberrant differentiation, and the resultant precursor lesion is designated melanocytic dysplasia. The vast majority of melanocytic nevi showing melanocytic dysplasia are terminal lesions that do not progress to melanoma. If melanoma is to develop via a precursor lesion, however, the nevus with melanocytic dysplasia is that precursor. When melanomas do develop, they develop focally within the precursor. The resultant primary melanoma itself does not follow a pathway of inexorable expansion of a population of melanoma cells in space and time. Rather, primary melanomas, with the exception of nodular melanoma, also evolve in a stepwise fashion. The first step, termed the radial growth phase, is characterized by the net enlargement of the tumor at its periphery, along the radii of an imperfect circle. Tumors in this stage of development show a characteristic pattern of growth within the epidermis and a distinctive form of invasion of the papillary dermis. Such melanomas are not associated with metastasis, and it is hypothesized that such tumors do not have competence for metastasis. For a melanoma to acquire competence for metastasis it must progress to the next step of tumor progression--the vertical growth phase. This lesional step is characterized by the appearance of a new population of cells within the melanoma, not an expansion of the cells forming the pre-existing radial growth phase. The net growth of the cells of the vertical growth phase is perpendicular to the directional growth of the radial growth phase. As a rule, the cells of the vertical growth phase grow in an expansile fashion, expansile as a balloon expands: a growth form characteristic of metastases.(ABSTRACT TRUNCATED AT 400 WORDS)