Cytophotomtric DNA contents of tumor cells in primary lesions and the corresponding metastatic lymph nodes were compared in 61 cases of gastric cancer to determine whether the DNA content remains stable during lymph node metastasis. The DNA distribution patterns were grouped into three types, according to the proportion of aneuploid cell population. Changes in DNA patterns between primary and metastatic lesions were evident in 36 of 61 patients (59.0%); in the remaining 25 (41.0%), the same DNA distribution patterns were noted for both lesions. In 33 of these 36, DNA pattern in the primary carcinoma was transformed into a more narrowly scattered one in the metastatic lesion of the lymph node. Mean and modal values and the frequency of cells over tetraploid (4c) or hexaploid (6c) were significantly higher in the primary lesion compared with findings in the metastatic lesions. This reduction in DNA content in the metastatic lesions was a more frequent occurrence in differentiated (18 of 23) than in undifferentiated adenocarcinoma (15 of 35) (P less than 0.01). Therefore, in primary lesions with a widely scattered DNA ploidy, the tumor cells with a smaller DNA ploidy frequently metastasized to lymph nodes, particularly in cases of a differentiated carcinoma. Such observations may be pertinent in future designing of treatment protocols.