One case of priapism occurred during heparin therapy for a previous surgical operation to the knee is reported. The association between heparin and priapism is often recognized; abnormal platelet aggregation could play a role in the pathogenesis of this side effect. A part from the kind of surgical treatment performed, priapism as a complication of heparin therapy might be functionally more severe than priapism due to other etiologies. Our observation would confirm this statement.