Numerical chromosomal abnormalities are a well known characteristic of human cancer, but no "chromosome-wide investigation" encompassing almost all of the chromosomes has ever been reported. Furthermore, although the multistep process of carcinogenesis is widely accepted in human cancer, the stepwise numerical aberration of chromosomes has never been addressed. Touch preparations of 24 (male 20, female 4) surgically resected gastric cancer tissue samples in various stages in terms of depth of invasion were analyzed by fluorescence in situ hybridization using centromere-specific probes including 17 chromosomes, 1-4, 6-8, 10-12, 15-18, 20, X, and Y. Microwave irradiation was performed to increase the sensitivity and specificity of the signal. The depth of the tumor invasion in the gastric wall and histological subtypes were recorded by viewing the histology of the adjacent portion. Numerical chromosomal abnormalities of chromosomes 1 and 2 were found most frequently and from the early stage of gastric cancer. The abnormalities observed were limited to chromosomes 1, 2, 4, and 20 in tumors invading to the middle layer of the submucosa of the gastric wall, but these became more extensive, involving almost all of the chromosomes investigated when the tumor had invaded beyond the proper muscle of the gastric wall. Centromeric numbers of chromosomes 3 and 18 were exceptionally stable even after the tumor progressed to advanced stage. These profiles of the sequential process of numerical chromosomal abnormality were similar in both mucocellular and tubular-type gastric cancer, but the prevalence was significantly lower in the mucocellular type (39.0% versus 68.0%). On the basis of fluorescence in situ hybridization analysis of 17 different chromosome centromeres in gastric cancer in various stages, we conclude that the earliest events in gastric carcinogenesis in terms of chromosomal abnormality occur in chromosomes 1 and 2 and that chromosomal numerical aberrations expand in a stepwise manner with cancer progression.