Low dose spironolactone reduces the risk of death from heart failure. We examined the effects of spironolactone on potassium homeostasis in a cohort of elderly patients with congestive heart failure (CHF). Eighteen patients > 70 years. mean 80.5 (+/-SD 6.3) with New York Heart Association CHF Grade II-IV were enrolled. All patients were commenced on 25 mg spironolactone daily. The dose was reduced to 12.5 mg daily when hyperkalemia (potassium > 5.0) occurred. A serum creatinine of > 150 mumol/l was defined as indicating renal impairment (RI). Blood pressure, pulse rate, urea, creatinine, Na+ and K+ were measured at baseline, day 2-5, day 28 and more often if clinically indicated. Nine of those recruited had RI. Baseline serum potassium was significantly higher in those with RI, mean 4.56 (+/-0.30) vs. 4.04 (+/-0.30) mmol/l (P<0.01). Six patients with RI developed hyperkalemia versus one of those with serum creatinine < 150 mumol/l (P<0.05). Serum K returned to normal in all patients when the dose of spironolactone was reduced to 12.5 mg daily with one exception in whom the medication was withdrawn. When spironolactone is prescribed to older patients with CHF, hyperkalemia appears more likely in those with RI. Halving the dose to 12.5 mg daily results in normalisation of serum potassium. Older patients commencing spironolactone therapy should have serum potassium monitored frequently, particularly in the presence of RI. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.