Melanoma cell lines initiated from metastases excised at the same time from multiple sites in a patient reflect a single clone origin. The similarities of the karyotypes of these lines depend upon the site of excision, the selective survival in vitro, and the time span in vitro before study. In the present investigation, eight short-term cultures of malignant melanoma from the same patient were analyzed cytogenetically, six after the second passage in vitro, one from the third, and one from the fifth passage. Karyotypes were similar on all eight cultures, each having chromosome abnormalities der(1)t(1q;?), der(3)t(1;3), and t(7;9). Four cell lines had an isochromosome 4q, which was missing in four lines. Two cell lines were missing del(6). In five of eight cultures there was an occasional i(16q) or t(16q;21q). In the final culture, made 6 months after the first seven, three cell types present included a type corresponding to that of seven previous cultures, a similar cell type with an abnormal chromosome 11, and a third cell type with a 3;19 translocation. Comparisons are made with similar studies in the literature and a hypothesis has been formulated to explain melanoma metastases as revealed by the cytogenetics of multiple lesions excised simultaneously.