BACKGROUND: Although many trials have been conducted to evaluate the feasibility and effectiveness of adjuvant chemotherapy (ACT) for patients with stomach cancer, the benefits of ACT remain unclear. Moreover, some authors have reported that ACT increased the incidence of second malignancy. The risk of second malignancy was evaluated in patients who underwent treatment for stomach cancer in the past 20 years at Osaka Medical Center for Cancer and Cardiovascular Diseases.METHODS: The study population consisted of 1925 patients who underwent gastrectomies for stomach cancer between the years 1978 and 1992 and who received follow-up examinations to check for second malignancies. They included 1114 patients who underwent surgery only (group A) and 811 who underwent surgery and received chemotherapy (group B). The observed incidence of second malignancy (O) was compared with the expected incidence (E), calculated by the person-year method, using data from the Cancer Registry in Osaka.RESULTS: The average follow-up period was 7.99 years. The total number of patients with a second malignancy was 127 (men, 97; women, 30); 72 patients had the second malignancy in digestive organs; 27 in respiratory organs; and 28 in other organs. The relative risks of a second malignancy in group A and B patients were 1.05 and 1.02 (differences between the two groups were not significant). The relative risks of a second malignancy in patients who received ACT with 5-fluorouracil, Tegafur and Uracil, and FT207 were 0.79, 1.01, and 1.06, respectively (differences between the groups were not significant).CONCLUSION: The risk of second malignancy after chemotherapy for stomach cancer was not high in comparison with the expected incidence. Adjuvant chemotherapy did not increase the risk of a second malignancy.