In a diabetic, who took a high dose of 5.85 g metformin daily for 75 days, urinary retention caused by a prostatic adenoma induced functional renal insufficiency and hyperlactacidemia, rapidly reversed with treatment. Plasma and urine levels of metformin were measured at the same time as lactate and pyruvate levels, until all returned to normal. Clinical and biological improvement occured at the same time. The case is discussed in the context of lactic acidosis in diabetics with functional or organic renal insufficiency, treated with metformin.