Severe lactate acidosis developed in nine diabetics on biguanide. When lactate acidosis was diagnosed all patients had reduced renal function, six being oligoanuric. Pre-existing chronic renal failure as a factor in the development of lactic acidosis was excluded in five patients, normal renal function being restored later. All patients were dialysed, seven surviving. This mortality rate is lower than that reported by others for biguanide-induced lactate acidosis. Rapid biguanide elimination by dialysis was demonstrated both in vitro and in vivo. The in vivo clearance of buformin was 83 +/- 43 ml/min (mean +/- SD, n = 4), that of phenformin 68 +/- 33 ml/min (n = 7). The main advantages of haemodialysis in the treatment of biguanide-induced lactic acidosis are rapid removal of toxic biguanides and excess lactate and the ability to administer sodium bicarbonate adequately without risking hypernatraemia and fluid overload.