For phase II studies in patients with solid tumors, the National Cancer Institute recommended that the starting dose of fludarabine phosphate be 20 mg/m2/day as a short intravenous infusion for 5 days every 21 days. Twenty-one patients with untreated, advanced, measurable colorectal carcinoma received fludarabine phosphate as a 30-minute infusion at a median dose of 25 mg/m2/day (range 15-35 mg/m2/day) for 5 consecutive days repeated every three weeks. Antitumor response was evaluated following two courses of therapy. No patient achieved complete or partial response. Minor regression of lung metastases occurred for less than 12 weeks in one patient. Therapy was generally well tolerated. Frequent toxicities included lymphopenia, mild nausea and vomiting, mucositis, and anorexia. One patient died of sepsis, bleeding, and progressive disease while she was severely myelosuppressed. Neurotoxicity was not observed in any patient. Fludarabine phosphate at this schedule and dose range is inactive against colorectal carcinoma.