Blood clotting, platelet aggregation, complete blood count and lipid profile were evaluated in 12 postmenopausal patients with advanced breast cancer. Patients under treatment with high-dose MPA were considered not at risk for thomboembolic disease and were given MPA orally, 800 mg/day, for at least 3 months. Laboratory investigations were performed prior to treatment with MPA then once weekly during the first month and every 2 weeks during the following months. PTT, TEG, antithrombin III and platelet adhesiveness underwent statistically significant changes, tending towards hypercoagulability, although, on the average, they did not exceed the upper normal range. The authors conclude that a clinically relevant thrombotic activity cannot be attributed to MPA at the administered oral doses in the absence of additional risk factors.