A 55 year old diabetic women treated with chlorpropamide and metformin for three years presented with acute oliguric renal failure and lactic acidosis from which she died. The plasma metformin level was very high suggesting that the lactic acidosis was caused by the drug. There were no contraindications to metformin therapy and renal function was normal three months previously. This case demonstrates that lactic acidosis can occur as a result of metformin therapy in the absence of pre-existing risk factors.