We analyzed routine serum potassium concentration measurements and conditions temporally associated with abnormalities in potassium concentration in outpatients on chronic hemodialysis (136 nondiabetics, 36 diabetics) and continuous ambulatory peritoneal dialysis (16 nondiabetics, 10 diabetics). The following potassium concentration frequencies were found: prehemodialysis, nondiabetics: normal 51.3%, severe hyperkalemia ( > 6.0 mmol/l) 10%, severe hypokalemia ( < 3.0 mmol/l) 0.3%; diabetics: normal 57.8%, severe hyperkalemia 8.7%, severe hypokalemia 0.5%. Peritoneal dialysis, nondiabetics: normal 73.7%, severe hyperkalemia 0.6%, severe hypokalemia 4.9%; diabetics: normal 72.5%, severe hyperkalemia 0.9%, severe hypokalemia 2.9%. Normokalemia and severe hypokalemia were significantly (χ2 40 mmol/l), metabolic acidosis (Tco2 30 mmol/l), was associated with a statistically higher risk of hyperkalemia