Thrombocytopenia (platelet count < 100,000/microl) is a rare side effect of tamoxifen (Nolvadex). We report a case of thrombocytopenia that developed 6 months after tamoxifen was given as an adjuvant treatment for breast cancer. The patient received no concurrent cytotoxic chemotherapy or radiation therapy. The thrombocytopenia resolved after the tamoxifen was stopped, reappeared promptly when the tamoxifen was restarted, and resolved again after the second withdrawal, at which time anastrozole (Arimidex) was substituted for the tamoxifen. The patient's platelet count remained normal for more than 6 months thereafter.