A Phase I trial of pirarubicin (4'-O-tetrahydropyranyl-doxorubicin) was undertaken to study its toxicity and to gain preliminary knowledge of its efficacy. The dose was escalated by increments of 10 from 30 to 70 mg/m2. Out of 20 patients, 19 were evaluable for toxicity and response to treatment. Hematologic toxicity was dose limiting and dose related. Other adverse effects included nausea and vomiting, hair loss, and stomatitis. No acute cardiotoxicity was encountered. In 2 patients with metastatic breast cancer who had not been pretreated with cytostatic agents, a partial remission was achieved lasting for 5 months. In 6 patients, tumor parameters did not change for a median of 3 months, and 11 patients suffered progressive disease.