Chromosome studies were performed on direct preparations, early passage cultures, and cell lines derived from melanocytic lesions of 37 patients. There were six congenital or common acquired nevi, six dysplastic nevi, one early primary melanoma (radial growth phase), three complex melanomas (RGP with foci of vertical growth phase), six advanced primary melanomas (VGP), and 26 metastases. The karyotype was normal in the six common nevi. A chromosomally abnormal clone with a single karyotypic alteration was found in two dysplastic nevi. All melanomas had clones with multiple cytogenetic changes. Nonrandom abnormalities involving translocations or deletions in the short arm of chromosome #1, either arm of chromosome #6, and/or extra copies of the short arm of chromosome #7 were present in all melanomas. These were not obviously associated with a particular stage of disease, except that the only nonrandom alteration in the early (RGP) melanoma involved chromosome #6. In four cases, cytogenetic data were available on both a primary melanoma and its metastases. In each instance there were common alterations (demonstrating the clonality of the disease), as well as additional changes in the metastases. Our findings indicate that demonstrable somatic genetic abnormalities increase in severity with clinical progression of melanocytic disease, but additional data are required to establish the significance of specific karyotypic changes (and the involved genes) in the clinical evolution of these disorders.