A phase I study of 4'deoxydoxorubicin (esorubicin) was performed on an every-21-day bolus intravenous (IV) schedule in 36 patients with advanced cancer. Thirty-four patients were evaluable for toxicity analysis. Toxicity included mild nausea, occasional local skin reactions, and mild to moderate alopecia. Myelo-suppression was dose limiting. Clinically evident congestive heart failure was not observed. However, two patients developed premature ventricular contractions. Overall, esorubicin was better tolerated than doxorubicin at equally potent doses. Although response analysis was not the primary objective of this phase I study, minor responses were observed in melanoma, breast cancer, lymphoma, and gastric cancer. On the basis of this study, a starting dose of 30 mg/m2 IV every 21 days is recommended for good-risk patients with escalation to 32.5 mg/m2 depending on bone marrow tolerance. For patients with poor bone marrow reserve, a starting dose of 25 mg/m2 every 21 days is recommended. Phase II trials with esorubicin in this dosage schedule are clearly warranted in a wide variety of metastatic neoplasms including a substantial population of patients who have not received prior chemotherapy.