Regional ploidy variations within individual tumors were analyzed by in-situ cytofluorometry of metaphase cells in Feulgen-stained paraffin sections, using 45 resected stomachs with early and advanced signet ring cell carcinomas. Aneuploid cells were found in one of 30 early cancers and in eight of 15 advanced cancers, and were almost always accompanied by diploid cancer cells in the mucosal part of the cancers. The diploid and the aneuploid cells were generally found to be distributed in different territories in the mucosa, and aneuploid foci were often included in the diploid area. These findings suggest the diploid origin of signet ring cell carcinomas and the occurrence of aneuploidy during the tumor development. Moreover, the aneuploid cells appeared to infiltrate beyond the mucosa more readily than the diploid cells; most of the aneuploid populations already invaded the extramucosal tissue, and the cancer cells infiltrating in the extramucosal tissue were predominantly aneuploid in six of the nine cancers with aneuploidy. Thus, it appears that the occurrence of aneuploid clones may accelerate the progression of signet ring cell carcinomas from early to advanced stages.