A phase I study of floxuridine circadian infusion was performed in 14 patients with advanced solid tumors (9 colonic, 1 gastric, 4 renal). The starting dose was 0.15 mg/kg/day for 14 days followed by a 14-day therapy-free interval. Sixty-eight percent of the daily dose was infused between 3pm and 9pm. The dose was increased by 0.025 mg/kg/day for each successive course. Eighty-one cycles of therapy were given for a total of 1134 days of treatment. The mean dose intensity was 0.868 mg/kg/day for the entire group. The highest dose achieved (maximum tolerated dose) was 0.325 mg/kg/day. The most frequent toxicity was diarrhea (4.9% of all courses) and nausea-vomiting (3.7% of all courses). These side effects were of a low grade and all were resolved without hospitalization. Our results suggest the circadian modulation of floxuridine infusion.