A 36-year-old woman presented with a 3-month history of increasing dyspnoea, culminating in respiratory failure due to paralysis of respiratory muscles. She required assisted ventilation. Subsequently, mild limb and neck weakness was noted. Muscle biopsy of the right vastus lateralis showed polymyositis. The patient died suddenly and the postmortem revealed polymyositis with predominant involvement of the diaphragm and intercostal muscles.