Expression of oncogenes in gastric cancer tissue was evaluated with immunohistochemical staining methods using monoclonal antibodies to products of the oncogenes. Rates of expression in gastric cancer tissue were 50% for c-myc, 72% for c-erb B2, and 56% for c-Ha-ras oncogenes. Expression of these oncogenes in gastric cancer was not correlated with the histologic differentiation. The c-Ha-ras oncogene was positive in 19 of 26 cases with lymph node and/or distant metastasis: the positive rate was significantly higher than in cases without metastasis. Results suggest that c-Ha-ras oncogene is related to the prognosis of gastric cancer. The rate of expression of c-myc and c-Ha-ras oncogenes in gastric cancer tissues was higher in the DNA aneuploid group than in the DNA diploid group. Expression of c-myc and c-Ha-ras oncogenes correlated with other prognostic factors such as DNA ploidy pattern and metastasis. These oncogenes can be used to evaluate prognosis of gastric cancer patients.