An 84-year-old man who was receiving oral anticoagulation therapy presented with complete lumbar plexus palsy caused by a massive psoas haematoma. Conservative treatment rather than drainage of the haematoma was undertaken, because of the risk of bleeding complications and mortality. At the one-year follow-up, the patient had no clinical signs of neurological recovery. The patient died 2 months later due to his concurrent medical problems. A high degree of suspicion is needed for the diagnosis because of the insidiously developing neurological deficit.