In the course of two and a half years we treated six diabetic women on account of lactate acidosis during concurrent biguanide administration. The patients were given an average dose of 290 mg Buformin/24 hours. Their mean age was 71 years. Three patients died, i.e. the mortality was 50%. All patients reported nausea, vomiting and abdominal pain. Two suffered from diarrhoea. Two patients suffered from renal failure and one from cardiac weakness. One patient was in coma. The mean lactate concentration was 14.7 mmol/l, pH on admission was 6.84. The patients were given on average 550 mmol bicarbonate. In two instances bicarbonate dialysis was used. The authors discuss the pathophysiology, clinical aspects and importance for treatment and prevention of lactate acidosis during biguanide treatment.