Metformin is widely used to treat diabetes. In rare cases, metformin may cause lactic acidosis, especially when administered during periods of dehydration. We present two cases of metformin-associated lactic acidosis (MALA). Both patients presented with metabolic acidosis, hypotension, renal failure and gastrointestinal symptoms. The use of continuous renal replacement therapy (CRRT) had a significant effect. Lactate and metformin can be removed effectively by haemofiltration. CRRT is widely available and a well-known treatment at intensive care units. MALA shows a mortality rate of about 30%.