Blood lactate and pyruvate were measured two to three hours after breakfast in 153 patients with maturity-onset (type 2) diabetes. Blood lactate was raised in 56% of all patients taking phenformin and in 35% of all patients taking metformin, while 75% and 35% respectively had raised pyruvate. Mean blood lactate level was raised at 2.2 mmol/1 (range 0.2-4.4 mmol/1) in the patients on phenformin and 1.7 mmol/1 (range 0.2-4.3 mmol/1) in those on metformin, with mean blood pyruvate levels of 0.122 mmol/1 (range 0.050-0.215 mmol/1) and 0.111 mmol/1 (range 0.038-0.259 mmol/1) respectively. In patients treated with a sulphonylurea drug alone mean blood lactate and pyruvate levels were not increased and were no different in those receiving sulphonylurea therapy in addition to a biguanide from those receiving a biguanide alone. Blood lactate did not correlate with any clinical or biochemical measurement in any of the treatment groups except for a correlation with serum creatinine in those patients taking metformin alone. The finding of raised blood lactate levels in diabetic patients treated with metformin suggests that this drug should be prescribed with caution.