In a phase II study, 18 patients with advanced gastric carcinoma were treated with intravenous 6-thioguanine. A 30-minute infusion of 55 mg/m2 (starting dose) was administered once a day for 5 consecutive days, the courses being repeated every 5 weeks. A median of 2 courses (range, 1-4) was administered. Among the 18 patients, 17 having measurable cancer and optimum follow-up were fully assessable for response. None of the patients achieved a complete or partial response. One patient achieved a transient minor response (15 weeks) of the primary gastric carcinoma but the metastatic carcinoma was unchanged. One patient had no change in his measurable carcinoma, and the other 15 patients had progressive disease while receiving intravenous-6-thioguanine. Myelosuppression, although frequent, was mild to moderate at these doses and did not result in significant morbidity. Nonhematologic toxicities were also mild. Our data suggest that intravenous 6-thioguanine given at this schedule is ineffective in previously untreated patients with advanced gastric carcinoma.