The limiting toxicity of low dose continuous infusion 5-fluorouracil (200-300 mg/m2/day) is often palmar-plantar erythrodysesthesia (PPE). PPE developed in 16/25 patients (exact 95% confidence interval of 42%-82%) with metastatic colon cancer enrolled in a phase II trial. In this trial, 5-FU was given continuously at a dose of 200 mg/m2/day until toxicity or progressive disease forced discontinuation. The first signs of the syndrome developed at a median of 2 months following infusion initiation and, unless treatment was interrupted, became progressively worse. The incidence of moderate to severe PPE was 71% in the 14 previously untreated patients (exact 95% confidence intervals of 42-92%). Seventy-eight percent of the responders in the no prior treatment group developed PPE. The incidence of moderate to severe PPE was only 27% in the 11 previously treated patients (exact 95% confidence intervals of 6-61%). The higher incidence of PPE in the previously untreated patients probably resulted from a longer total infusion time (median = 7.3 months) than the previously treated (median = 4.5 months). The longer infusion time in turn was a result of the higher response rates (64 vs 18%) in the previously untreated versus treated groups. Five previously untreated patients who developed PPE received 50 or 150 mg of pyridoxine/day when moderate PPE changes were noted. Reversal of PPE without interruption of the 5-FU was seen in 4/5 patients. Four of these patients who received pyridoxine had responded to 5-FU treatment. No adverse affect of pyridoxine on clinical response was noted.(ABSTRACT TRUNCATED AT 250 WORDS)