Comorbidity has important implications in older breast cancer patients. Whether elderly patients are more likely to have a decreased tolerance to antineoplastic agents is still a question of debate. Chemotherapy-naive women with metastatic or locally advanced breast cancer aged > or = 68 years entered a phase II trial of Mitoxantrone given day 1 q 21 days. The dose of the drug was dependent on performance status (PS) and number of "minor" comorbid conditions: patients with ECOG PS = 2 and/or > or = 2 comorbid conditions received 10 mg/m2 and those with PS < 2 and < 2 comorbid conditions received 14 mg/m2. Twenty-seven patients, median age 77 years (range 68-86), received a median number of 5 courses (range 1-9). All 27 patients were evaluable for toxicity and 23 cases (patients receiving at least 3 courses) were evaluable for response. Partial response was observed in 6/23 cases (26%) and median duration of response was 6 months (range 3-9). Median overall survival was 8 months (range 2-34). Among cases receiving 10 mg/m2, 4 PR, 4 SD and 2 P were observed and in cases treated with 14 mg/m2, 2 PR, 5 SD and 6 P occurred. Treatment was well tolerated and no grade 4 toxicity was encountered. Mitoxantrone is an effective and well-tolerated regimen in elderly breast cancer patients presenting with comorbid conditions.