Hepatic intra-arterial (HIA) infusion of floxuridine (FUDR) via an implanted pump has shown promise in the treatment of colorectal cancer metastasized to the liver. However, the potential benefit of this therapy may be offset by the high incidence of treatment-limiting biliary toxicity. Although weekly HIA bolus of fluorouracil (5-FU) is effective against metastatic colorectal cancer to the liver with no biliary toxicity, it is limited by systemic side effects. In December 1986, we began a phase II trial of alternating HIA FUDR and 5-FU via the implanted pump in an attempt to extend the duration of treatment by obviating the limiting biliary (FUDR) and systemic (5-FU) drug toxic effects. Patients received continuous HIA FUDR at 0.1 mg/kg of body weight per day on days 1 through 8 followed by an HIA bolus of 5-FU at 15 mg/kg given via the pump sideport on days 15, 22, and 29, with the cycle repeated every 35 days. Sixty-eight patients were enrolled in this trial, and 64 were fully evaluable. Of the 64 patients, 30 (47%) previously had received chemotherapy. Major response (complete response plus partial response) was observed in 32 (50%) of 64 patients, and the median survival from pump implantation in all patients was 22.4 months. In contrast to the experience with the single-agent HIA FUDR regimen, no patient had treatment terminated because of drug toxicity. Alternating HIA FUDR and 5-FU has efficacy similar to that of HIA FUDR given alone, but when closely monitored and adjusted appropriately, is not associated with toxic effects requiring treatment termination.