Fosinopril sodium is the first of the phosphinic acid class of angiotensin-converting enzyme inhibitors (ACEI). It is used as an antihypertensive agent, but differs from other ACEI in its dual routes of excretion (liver and kidney), and less incidence of hyperkalemia and cough. We conducted a study in known chronic hemodialysis patients who developed interdialytic hyperkalemia in spite of other treatments to control hyperkalemia. We used fosinopril in this group of patients to assess the effect of fosinopril on serum potassium (K) levels. Twenty-four patients were given fosinopril 10 mg at 18:00 h daily for 8 weeks. K levels were measured before and after each dialysis treatment. Interdialytic weight gains were recorded. The average pretrial potassium level was 6.57 mmol/l (± 0.47), and the posttrial level was 5.34 (± 0.76); p ≤ 0.0001. No statistically significant interdialytic weight gain or reduction occurred. No first dose hypertensive effect or cough were reported. We have found the use of fosinopril to be successful in lowering predialysis serum K levels in hyperkalemic dialysis patients.