One hundred and thirty-two previously untreated patients with metastatic adenocarcinoma of the gastrointestinal (GI) tract were randomized to receive either a 120-hr infusion of 5-fluorouracil (5FU) with mitomycin-C or mitomycin-C alone. Superiority of the combination treatment was demonstrated with remissions in 30 out of 82 (37%) patients versus 9 out of 50 (18%) with the single drug treatment (P = 0.02). The median survial with 5FU--mitomycin-C was 29 weeks, as opposed to 20 weeks with mitomycin-C alone (P = 0.03). The combination produced significantly more severe myelotoxicity than the single drug, and jaundiced patients experienced more myelosuppression than non-jaundiced patients with both treatments.