Aneuploidy and hyperploidy are often detected in malignant melanoma by cytogenetic analysis and flow cytometric analysis of DNA content. To determine the ploidy of cells in surgical specimens of melanin-producing tumors of Japanese patients, we performed fluorescence in situ hybridization (FISH) using touch smear technique to count the number of chromosomes 18 and X + Y in interphase nuclei using alpha-satellite DNA probes, D18Z1, DXZ1 and DYZ3. A normal melanocyte strain showed two D18Z1 and two [DXZ1+DYZ3] signals per nucleus, indicating 2N, and a malignant melanoma cell line showed 4 per nucleus, indicating 4N, consistent with results of cytogenetic and flow cytometric analyses. Therefore we employed this FISH method to analyze ploidy of surgical specimens. Specimens obtained from 8 patients with nevus cell nevus showed 2 FISH signals per nucleus. On the other hand, in all specimens obtained from 8 patients with malignant melanoma (6 primary and 2 metastatic melanoma), 65-90% of cells exhibited 4 signals per nucleus, indicating 4N. Histopathologically, 50-70% of cells were identified as malignant melanoma cells, indicating that our FISH method is effective to detect melanoma cells in tissue. We also analyzed allelic loss of the p53 gene by FISH with a p53 locus-specific probe and mutation of the p53 gene by immunostaining since mutation and deletion of the p53 gene may cause hyperploidy. All specimens except one obtained from a case with young-onset metastatic melanoma exhibited no allelic losses or negative p53 staining, showing the p53 gene was intact. These results indicate that tetraploidy, not caused by p53 mutation or deletion, is commonly found in malignant melanoma of Japanese patients. It is also suggested that there is no positive relationship between tetraploidy and poorer prognosis, and mutation and allelic loss of the p53 gene might be markers of aggressive form of malignant melanoma.