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Objective: The antidiabetic drug metformin has been associated in a small number of patients with lactic acidosis, a serious condition with a poor prognosis. However, because of lack of data, the prognostic significance of hyperlactataemia in metformin-treated patients is not known., Methods: Data were collected from 49 metformin-treated patients with lactic acidosis (arterial lactate level > or = 5 mmol/L and blood pH < or = 7.35) and available plasma metformin concentration data to investigate the association of arterial lactate levels and plasma metformin concentrations with mortality., Results: The overall mortality rate in this patients sample was 45% and the median arterial lactate level was 13.1 mmol/L. Median lactate levels were similar in patients who survived (13 mmol/L) and those who died (14.3 mmol/L), whereas the median plasma metformin concentration was 3 times higher in patients who survived (20.6 mg/L versus 6.3 mg/L)., Conclusion: In this, the largest series of metformin-treated patients with lactic acidosis yet reported, 55% of patients survived and these patients had a median arterial lactate level of 13.1 mmol/L. Neither arterial lactate levels nor plasma metformin concentrations were of prognostic significance in relation to mortality in this sample of metformin-treated patients with lactic acidosis. Death in these patients appeared instead to be associated with other hypoxic disease or underlying ill health. These observations suggest that accumulation of metformin may not be as significant with respect to high arterial levels of lactate and their effects as has been traditionally thought.