We present the case of a patient with classical protein C deficiency presenting with acute priapism during warfarinization for thrombophlebitis. Priapism is a well‐recognized complication of a number of conditions including sickle cell disease and haematological malignancies, but to our knowledge it has not previously been reported in association with protein C deficiency. This case highlights the potential dangers of initiating oral anticoagulant therapy using conventional loading dose regimens in patients with protein C deficiency.