The interphase cytogenetics in formalin-fixed and paraffin-embedded gastric cancer tissues were examined by fluorescence in situ hybridization (FISH) with alpha-satellite DNA probes. Two gastric carcinoma cell lines, TMK-1 and MKN-28, were first analyzed cytogenetically. Of 25 TMK-1 cell karyotypes, chromosome 7 showed trisomy and chromosome 17 showed disomy in 18 cells. Most MKN-28 cells showed disomy of both chromosomes 7 and 17. Suspensions of singly isolated TMK-1 and MKN-7 cells were obtained from the cultured cells, and from paraffin-embedded tissue specimens fixed with formalin for 0, 1, 3 and 5 days obtained from xenotransplanted tumors in nude mice. The numbers of chromosomes 7 and 17 analyzed with the karyotypic preparations coincided well with those determined by FISH, even in the paraffin-embedded specimens. The number of tumor cells showing no signals, however, increased in the specimens after 5 days formalin fixation. In 10 surgically removed gastric carcinomas, the predominant signal number for chromosomes 7 and 17 in the cells of paraffin-embedded tissues was two (disomy), except in one papillary carcinoma, which was trisomic for chromosome 7. Large subpopulations (more than 20%) showing trisomy were found in four cases for chromosome 7 and in five cases for chromosome 17. A higher frequency of trisomy was found in well differentiated than in poorly differentiated carcinomas. These findings suggest that the FISH technique is a useful tool for detecting chromosomal aberrations in gastric adenocarcinoma cells, even in paraffin-embedded specimens, as long as the tissues are fixed with formalin for an appropriate time.