Experimental studies have demonstrated that thalidomide has anti-tumor activity mediated by blockage of angiogenesis, with clinical efficacy in multiple myeloma, glioblastoma multiforme, and renal cell cancer. We investigated the therapeutic activity and toxicity of thalidomide in patients with progressive metastatic breast cancer pretreated with chemotherapy. Inclusion criteria were metastatic breast cancer in progression of disease after at least two lines of chemotherapy, age > or = 18 years, performance status < or = 2, and adequate hematologic, renal, and hepatic functions. Twelve patients entered the study, eight of whom were pretreated with three or more lines of chemotherapy (66.7%). Thalidomide was well tolerated: the most common side effects were constipation and somnolence (58.3% of patients). No objective response or durable stable disease was observed. Median time to progression and median overall survival were 8 weeks (range, 4-10 weeks) and 16 weeks (range, 8-54 weeks), respectively. In conclusion, thalidomide is an ineffective treatment in patients with progressive metastatic breast cancer heavily pretreated with chemotherapy.