Fluorescence in situ hybridization (FISH) to sections of formalin-fixed paraffin-embedded archival tissues allows the detection of gene or chromosome copy number changes in interphase cell nuclei within the histological context and thus may be of particular interest in tumor pathology. In this report, we describe the application of FISH to thick (15 microns) paraffin sections of 7 primary cutaneous malignant melanomas. A chromosome 7-specific centromeric DNA probe was used to detect numerical aberrations of chromosome 7. By optical sectioning using confocal laser scanning microscopy (CLSM) only complete, uncut interphase cell nuclei were scored. The mean percentage (+/- SEM) of melanoma cell nuclei with three hybridization spots was (20.7 +/- 2.8)%; (6.8 +/- 1.0)% of nuclei showed one spot and (5.0 +/- 1.2)% four or more spots. The frequency distribution of spot numbers among melanoma cell nuclei and normal keratinocyte nuclei was significantly different (chi(2) = 176.8, df = 5, p < 0.001). Trisomy 7 was detected in all 7 cases analyzed, mostly associated with monosomy 7 or polysomy 7. The approach used in our study and the data obtained could be useful for further studies designed to investigate a possible involvement of chromosome 7 in melanocytic tumor progression.