The intratumor DNA ploidy distribution pattern in 74 undifferentiated carcinomas of the human stomach was analysed by flow cytometric measurements of DNA for stepsectioned specimens taken from throughout the entire tumor. 52 cases showed intratumor DNA ploidy heterogeneity (70.3%), and a conspicuous difference between intramural and transmural carcinomas was seen with regard to the intratumor DNA ploidy distribution pattern as well as prognosis. In the case of transmural carcinomas, the following findings were noted: prognosis was significantly worse, predominant aneuploidy was seen more frequently, the frequency of intratumor DNA ploidy heterogeneity as well as DNA content was significantly increased, and predominant diploidy exhibited a worse prognosis. These were all in contrast to the findings for intramural carcinomas.