Exemestane was administered orally to postmenopausal women with advanced/recurrent breast cancer at a dose of 10 mg/day or 25 mg/day once daily for more than 8 weeks in order to evaluate the drug's anti-tumor effects and safety in a dose-finding study. The response rate (CR + PR) in the 10 mg and 25 mg group was 25.0% (8/32) and 31.4% (11/35), respectively, demonstrating no significant differences between the two groups, yet a higher efficacy rate was observed in 25 mg group. The efficacy rate in hormone-treatment-resistant patients within the 10 mg and 25 mg groups was 14.3% (3/21) and 26.1% (6/23), respectively, demonstrating more than a 20% response rate in 25 mg group. Incidences of the adverse events of which relevance to the drug could not be excluded were 30.6% (11/36) in the 10 mg group. 13.9% (5/36) in the 25 mg group and 22.2% (16/72) in the total group. The major adverse events were, hot flashes, numbness of the limbs, nausea, headache etc. Abnormal findings in clinical laboratory tests were as follows: ALP increase; GOT increase; GPT increase; gamma-GTP increase; total cholesterol increase; urinary sediment present. Abnormal findings in endocrine function were as follows: aldosterone decrease; testosterone.cortisol.DHEA-S decrease. But discontinuation due to abnormal laboratory findings was not found. No abnormal findings in physical tests were observed. A significant decrease in plasma estrogen concentration at week 4 was observed in both the 10 mg and 25 mg groups compared with baseline. These low levels were maintained throughout the study period. On the basis of these results, the efficacy of exemestane 25 mg/day was verified to be slightly higher than 10 mg/day. In addition the safety profile had no major adverse events to notice. In these patients with advanced/recurrent breast cancer, 25 mg/day was recommended as the most appropriate dose to be used clinically.