Two cases of diabetic patients on phenformin (phenethylbiguanide) hydrochloride therapy in metabolic acidosis due to lactic acidosis are described. Each had hypotension due to myocardial and mesenteric infarction respectively. Significant ketosis was not present. Altered lactate metabolism has been described in diabetic patients treated with phenformin hydrochloride. It is possible that the hyperlactatemia of vascular collapse may have been magnified by a biguanide block in oxidative metabolism. However, it is equally possible that the association was entirely fortuitous, particularly since lactic acidosis is being described with increasing frequency in patients not receiving phenformin hydrochloride.