Blood lactate concentration has been measured at intervals over a twelve hour period in four maturity-onset diabetics with retinopathy treated by sulphonylurea and phenformin therapy. Despite moderately good control of blood glucose, all four patients had markedly elevated concentrations of blood lactate (12h means of 2.1-2;4 mmol/1) with concentrations greater than 3 mmol/1 at some time during the day. Hepatic function was normal in all patients and in all four patients plasma creatinine was 80-85 mumol/1, (normal range 60-125 mumol/1) so that the usual indications for not using biguanides were absent. It is concluded that diabetics with retinopathy constitute a group of patients with an increased risk of phenformin-associated lactic acidosis, and that plasma creatinine estimation may be unreliable in assessing whether patients are suitable for phenformin therapy.