This case report presents a patient who developed severe life-threatening hyperkalemia following combined treatment with an ACE-inhibitor and the aldosterone-antagonist spironolactone for his congestive heart failure and who also suffered from pre-existing moderate renal failure. The pathophysiological reasons and the clinical evidence for the treatment of congestive heart failure with a combination of ACE-inhibitors and aldosterone-antagonists are discussed in details. The reasons for the occurrence of hyperkalemia in this patient and how spironolactone should be prescribed in patients with congestive heart failure are also elucidated.