Seven of 142 cases (5%) of Mediterranean spotted fever admitted to hospital in Marseilles in 1983 and 1984 were severe. All of them were confirmed by specific laboratory tests. The disease resembles Rocky Mountain spotted fever with purpuric exanthem, confusion, renal insufficiency, hypoxaemia, thrombocytopenia, hyponatraemia and hypocalcaemia. Two patients died. The predisposing factors for severity were old age, alcoholism, heavy smoking, respiratory insufficiency and glucose-6-phosphate dehydrogenase deficiency.