A 62-year-old male patient, suffering from a hypertensive cardiopathy, an arteriopathy of the lower extremities and a type II insulin independent diabetes had a prothrombin time ratio R of 2.15 (therapeutic range: 2.1-1.45) during a daily treatment of 4-6 mg acenocoumarin. On admission to the psychiatric hospital for depression, a mianserin treatment was commenced, leading to a sharp modification in the PT ratio R, in effect that the acenocoumarin treatment had to be increased to keep R within the therapeutic range. The possible mechanism of this interaction between mianserin and acenocoumarin is discussed, taking into account the possible role of the comedicated amiodarone.