Ten patients with advanced cancer were treated in a phase-I trial with monthly cycles of a 5-day infusion of vinblastine. Serum drug levels were relatively constant after 24 h and there was no correlation between drug level and myelosuppression or toxicity. Previously undescribed toxicities were observed, including one hypersensitivity reaction and three cases of severe, only slowly reversible, sensorimotor neuropathy. Two patients showed objective tumor regression during the vinblastine infusion therapy.