Encouraging results have recently been reported for studies using folinic acid in combination with 5-fluorouracil (5-FU) in the treatment of patients with gastrointestinal (GI) malignancies. Thirty-six patients with advanced colorectal cancer with unequivocal evidence of progression while treated with fluoropyrimidines were treated with a six-day continuous infusion of 500 mg/m2/d of folinic acid initiated 24 hours before a five-day course of 5-FU administered as an intravenous (IV) bolus of 370 mg/m2/d. An initial dose of 250 mg/m2/d of 5-FU was used in patients previously treated with ionizing radiation and/or a nitrosourea. Three objective partial responses were observed. The overall median duration of survival was 8.1 months. Toxicity was acceptable and not in excess of that expected for 5-FU alone.