A 37 year old man with Behçet's disease who was maintained on prolonged corticosteroid therapy, developed diabetic ketoacidosis and pneumonia. Secondary infection with mucor intervened with abscess formation cured by pneumonectomy. The association of Behçet's disease and mucormycosis has not been previously reported, although diabetes mellitus was almost certainly the predisposing cause. Surgical treatment offers the best chance of survival in similar cases.