The aim of this study was to determine the characteristics of metformin elimination by dialysis. For this purpose we report the kinetic parameters during dialysis and the metformin clearance (i.e. dialysance) in four patients presenting with lactic acidosis which occurred on metformin therapy. We also studied metformin elimination in two chronically hemodialyzed diabetic patients inadvertently maintained on metformin therapy and in two chronically hemodialyzed non-diabetic patients who took a single dose of metformin before a dialysis session. Analysis of plasma concentration-time curves showed a biphasic pattern of metformin - elimination, according to a two-compartment model. We demonstrate that metformin may be removed even after reaching an equilibrium between blood and dialysate levels in a recirculating system, suggesting a storage of metformin in a deep compartment with a gradient of concentration between this compartment and the blood. Lastly, metformin dialysance appears satisfactory (68 ml/min) even in the case of relatively low blood flow; this value reached 170 ml/min under good hemodynamic conditions. In conclusion, hemodialysis efficiently removes metformin and corrects metabolic acidosis in patients with metformin-induced lactic acidosis.