Study Objective. To evaluate the frequency of venous thromboembolism (VTE) in patients with advanced androgen-independent prostate cancer who were treated with docetaxel alone or in combination with thalidomide. Design. Retrospective analysis of a randomized phase II trial. Setting. National Institutes of Health clinical research center. Patients. Seventy men, aged 50-80 years, with advanced androgen-independent prostate cancer. Intervention. Each patient received either intravenous docetaxel 30 mg/m2/week for 3 consecutive weeks, followed by 1 week off, or the combination of continuous oral thalidomide 200 mg every evening plus the same docetaxel regimen. This 4-week cycle was repeated until there was evidence of excessive toxicity or disease progression. Measurements and Main Results. None of 23 patients who received docetaxel alone developed VTE, whereas 9 of 47 patients (19%) who received docetaxel plus thalidomide developed VTE (p=0.025). Conclusion. The addition of thalidomide to docetaxel in the treatment of prostate cancer significantly increases the frequency of VTE. Clinicians should be aware of this potential complication when adding thalidomide to chemotherapeutic regimens.