BACKGROUND In a previous trial in which methotrexate and N-phosphonacetyl-L-aspartate (PALA) were used to modulate 5-fluorouracil (5-FU), four of six patients could not tolerate treatment at the 600 mg/m2 5-FU dose level because of mucositis, diarrhea, and a decrease in performance status. The current study examines the ability of uridine rescue to prevent such toxic effects in the same regimen and, thereby, allow additional dose escalation of 5-FU. METHODS Twenty-nine patients with advanced malignant neoplasms received PALA and MTX, each at 250 mg/m2, followed 24 hours later by increasing bolus doses of 5-FU (600-750 mg/m2) with a leucovorin rescue (10 mg orally every 6 hours for eight doses) and uridine rescue (3 g/m2/hour, for a 72-hour infusion, 3 hours on, 3 hours off). Treatment was repeated weekly with either 2 weeks on, 2 weeks off, or 3 weeks on, 1 week off. RESULTS Mucositis, which occurred in 4 of 12 patients treated at the 750 mg/m2 5-FU dose level, was the only significant chemotherapy-induced toxic effect. However, uridine-related central venous catheter complications (cellulitis in six patients and superior vena cava syndrome in one patient) precluded additional treatment on this protocol. CONCLUSIONS In the current regimen, uridine allowed dose escalation of 5-FU to 750 mg/m2, which some patients tolerated on a 3-week on, 1-week off schedule. Because of the vascular toxic effects associated with intravenous uridine, the authors recommend additional studies with oral uridine to determine whether the increase in 5-FU dose that uridine allows is associated with improved response rates.