This report describes the occurrence of asymptomatic hyperkalemia induced by the sequential administration of metoprolol and carvedilol in an 81-year-old man with type II diabetes and stable stage III renal insufficiency. The potassium level rose to 5.6-5.7 mEq/L with metoprolol and normalized when the agent was discontinued. However, the potassium level rose again to 5.6 mEq/L after the administration of carvedilol but the level normalized by halving the dose. The observations of hyperkalemia induced by two different β-blocker drugs in the same patient confirm that this side effect is common to all β-blocker drugs.