Attempts to prevent peritoneal carcinomatosis after surgery for gastric cancer by intraperitoneal administration of anticancer drugs have not been successful, largely because the drugs are not retained in the peritoneal cavity. We have assessed the prophylactic efficacy of a delayed-release preparation--mitomycin adsorbed onto activated charcoal (M-CH). 50 patients with gastric cancer and serosal infiltration were randomly assigned intraperitoneal treatment with M-CH (50 mg mitomycin intraoperatively) or no anticancer prophylaxis (control). Survival rates for the 3 years of follow-up were significantly higher among the 24 M-CH recipients (1 was lost to follow-up) than among the 25 controls (p less than 0.01). There were significant differences in survival between the groups at 1.5 years after randomisation (difference 34.6% [95% confidence interval 8.5-60.8%]; p less than 0.01) and at 2.0, 2.5, and 3.0 years (41.7% [14.2-69.1%]; p less than 0.005). The concentration of mitomycin was significantly higher in peritoneal exudate than in plasma for 24 h after drug administration. Side-effects were slight and well tolerated. Thus, peroperative intraperitoneal treatment with M-CH seems to improve survival after gastrectomy for gastric cancer, presumably by a prophylactic effect on peritoneal recurrence.