A novel sequential administration schedule of PALA (N-phosphonoacetyl-L-aspartate) and thymidine to enhance the cytotoxic effect of 5-fluorouracil (5FU) was tested in 36 patients with advanced gastric cancer and 21 patients with advanced poorly differentiated (anaplastic) colorectal cancer. The potency of 5FU was dramatically increased as indicated by the observation of dose-limiting leukopenia at less than one tenth the maximum tolerated dose of 5FU when given as a single agent by intravenous bolus technique. Twenty-five percent of gastric cancer patients and 33% of colorectal cancer patients experienced an objective tumor response, including three patients with complete response. However, response duration was brief (median, 6 months), and there were four treatment-related fatalities due to severe and unpredictable leukopenia leading to sepsis. Survival was short with a median of 6 months for gastric cancer patients and 3 1/2 months for colorectal cancer patients. We conclude that therapeutic index of 5FU was not improved by the addition of PALA and thymidine in this patient population based on considerations of objective tumor response rate, patient survival, and toxicity.