Adjuvant therapy for breast cancer with tamoxifen is suggested to be of benefit to both women with negative and women with positive axillary nodes, and treatment lasting several years is currently being investigated. Venous thrombophlebitis may complicate tamoxifen treatment at a rate of approximately one per 800 treatment-years. To explore the possible mechanisms of this effect, we evaluated changes in antithrombin III levels, fibrinogen levels, and platelet counts in 140 postmenopausal women with surgically resected breast cancer who were disease free and participating in a double-blind, placebo-controlled, randomized toxicity study of tamoxifen. Antithrombin III levels, elevated at baseline evaluation, decreased in tamoxifen-treated subjects at 6 months, but no subject exhibited a drop to clinically significant levels. Fibrinogen levels decreased 15% (0.4 g/L) in tamoxifen-treated subjects at 6 months. Platelet counts decreased 7% to 9% from baseline to evaluations at 3, 6, 12, 18, and 24 months in tamoxifen-treated subjects. While these changes do not explain the possible small thrombophlebitis-promoting effect of tamoxifen, the decrease in fibrinogen levels might be expected to be associated with a decreased risk of arterial thrombosis.