Eleven diabetic patients were admitted to Groote Schuur Hospital during the past year with severe lactic acidosis; 8 patients recovered and 3 died. All were middle-aged or elderly, taking phenformin, and suffering from a complicating disorder including Gram-negative septicaemia, shock, chronic hepatic disease and chronic renal disease. In some the complicating feature was not in itself serious, either a short anaesthetic or an upper respiratory tract infection, or minor alcohol ingestion. The purpose of this article is to draw attention to a serious complication of phenformin therapy in the patient who has any predisposition to the accumulation of lactic acid.S. Afr. Med. J., 48, 2021 (1974).