BACKGROUND An optimal treatment schedule of 5-fluorouracil (5-FU) remains to be clarified. METHODS A randomized study was conducted to investigate schedule-dependent thymidylate synthase (TS) inhibition by 5-FU in 16 patients with gastric cancer who underwent surgical resection. Surgical specimens of tumor, normal gastric mucosa, and regional lymph nodes were obtained 12 hours after administration of 5-FU either as a continuous infusion (1000 mg/m2 for 48 hours) or as a bolus injection (500 mg/m2 x 2 in 48 hours). RESULTS The total TS activity (567.8 +/- 294 fmol/mg protein) and the rate of TS inhibition (74.7 +/- 23.1%) in cancer tissues were significantly higher in the continuous-infusion group than in the bolus-injection group (228.5 +/- 104.6 fmol/mg protein and 48.8 +/- 12%, respectively). Likewise, the total TS activity (807.4 +/- 440.3 fmol/mg protein) and the rates of TS inhibition in lymph nodes (72.3 +/- 17.1%) and in normal gastric mucosa (85.1 +/- 12.2%) were significantly higher in the continuous-infusion group than those in the bolus-injection group (232.4 +/- 142.3 fmol/mg protein and 53.6 +/- 17.0% in lymph nodes and 46.5 +/- 14.3% in normal gastric mucosa, respectively). There was a significant correlation between the total TS activity and TS inhibition. CONCLUSIONS Continuous infusion of 5-FU provides a superior antimetabolic effect in the treatment of gastric cancer, which may lead to a superior antitumor effect.