O-6-Methylguanine DNA methyltransferase (O-6-MGMT) reverses DNA alkylation damage produced by alkylating agents. O-6-MGMT is also a major determinant of cellular resistance to adjuvant chemotherapy with alkylating drugs. O-6-MGMT activity was measured in samples from patients with gastric cancer, including tumor, adjacent normal appearing mucosa, and peripheral blood leukocytes (PBL). O-6-MGMT activity of PBL from healthy individuals was evaluated as control. There was no significant difference between controls and patients for O-6-MGMT activity in PBL. O-6-MGMT activity was significantly increased in tumor tissue. Tumor O-6-MGMT activity was found to be independent from tumor subgroups and tumor grade. A positive correlation was determined between O-6-MGMT activity in tumor and in circulating PBL. The results indicate that O-6-MGMT, a defense protein against alkylating agent-mediated carcinogenesis, increased in gastric tumors. This may explain the low response rate to drug combinations, including chloroethylnitrosoureas, exhibited by patients with gastric cancer.