Irinotecan hydrochloride has been administered to patients with breast cancer resistant to anthracyclines and/or taxanes in our department. A retrospective analysis of the efficacy and toxicity of irinotecan therapy was conducted to clarify its clinical usefulness. A total of 35 consecutive patients with advanced or recurrent breast cancer were treated with irinotecan between June 1996 and March 2002. The patients ranged in age from 37 to 66 years old (median, 52). The most frequent metastatic lesion was in the liver. The number of previous chemotherapy was 2 to 7 regimens (median, 3). Ninety-one percent and 97% of the tumors were anthracycline- and taxane-resistant, respectively. The weekly dose of irinotecan was 40-160 mg/body (median, 100), and the total dose was 40-6, 110 mg/body (median, 840). An objective response rate of 6% and a clinical benefit rate of 23% were obtained. The median time-to-progression and overall survival were 3 months and 8 months, respectively. Severe toxicity (grade 3 or 4) was observed in 34% of the patients for a decrease in the white blood count, in 26% for neutropenia, in 17% for nausea/vomiting and in 6% for diarrhea. Although this study suggests that irinotecan is a clinically useful treatment of anthracycline- and/or taxane-resistant breast cancer, its anti-tumor effect was not satisfactory. The activity of first-line irinotecan therapy or the combined use of irinotecan with other agents should be investigated in clinical studies.