A Phase I trial of Taxol administered as a 120 h infusion once every 3 weeks was conducted in 20 patients with advanced cancer. The initial dose was 5 mg/m2/d (25 mg/m2 total dose) and patients received 10 mg/m2/d, 25 mg/m2/d, 30 mg/m2/d and 36 mg/m2/d. Forty-four courses of taxol were administered and all patients were evaluable for toxicity. Grade 4 leukopenia was the dose limiting toxicity observed in 50% of patients treated with 36 mg/m2/d. Significant mucositis was also observed at 30 and 36 mg/m2/d. All toxicity resolved within three weeks of treatment and no cumulative toxicity was observed. No neurotoxicity or cardiotoxicity was observed and no episodes of hypersensitivity reaction were noted. We conclude that 30 mg/m2/d is an appropriate dose for phase II testing of this schedule.