1. We describe a patient who developed deep vein thrombosis after commencing treatment with the synthetic androgen, mesterolone (Pro-Viron). 2. To investigate the potential thrombogenic action of this drug, a 21 day course of mesterolone (100 mg/day) was given to nine healthy male volunteers. 3. No significant change after treatment occurred in any of the blood tests performed (clotting times, Factor VIII coagulant activity, Factor VIII related antigen, antithrombin III activity, fibrinogen, fibrinogen-fibrin degradation products, plasminogen, euglobulin lysis time, urokinase sensitivity, platelet count, haematocrit, whole blood viscosity and plasma viscosity). 4. We conclude that in a conventional dose taken for 3 weeks mesterolone does not produce a consistent measurable prothrombotic state, nor does it enhance fibrinolysis.