BACKGROUND It is common practice to utilize a series of different hormonal agents in the treatment of postmenopausal women who, despite disease progression, continue to be candidates for hormonal therapy on a clinical basis. Letrozole is a new highly selective and potent aromatase inhibitor. There are limited data on third-line hormonal therapy in general, and this study was undertaken to evaluate letrozole in this context. METHODS A randomized trial involving two independent Phase II trials of two letrozole dosage levels, 0.5 mg and 2.5 mg per day, was performed. Eligibility requirements included failure on two prior hormonal therapies and measurable or evaluable disease. RESULTS Ninety-one patients, 46 receiving 0.5 mg and 45 receiving 2.5 mg of letrozole per day, were assessable for response. At the lower dose, 9 patients (20%) achieved an objective response; 6 patients (13%) had this documented on 2 occasions separated by 3 months. At the higher dose, 10 patients (22%) achieved a response; 8 patients (18%) had this documented on 2 occasions separated by 3 months. The median times to progression were 97 days for the lower dose and 154 days for the higher dose. Toxicity was considered acceptable. CONCLUSIONS Letrozole has definite antitumor activity as third-line hormonal therapy for women with metastatic breast carcinoma at doses of 0.5 and 2.5 mg per day. It is an effective and generally well-tolerated hormonal agent.