Four patients with the acquired immune deficiency syndrome (AIDS) and persistent unexplained hyperkalemia were studied. Testing with cosyntropin (0.25 mg intravenously) revealed normal baseline and stimulated cortisol levels and adequate aldosterone stimulation. The baseline aldosterone level was low for the degree of hyperkalemia. Renin/aldosterone stimulation testing was performed by intravenous injection of 80 mg of furosemide followed by four hours of upright posture. This study showed low baseline renin and aldosterone levels and inadequate renin and aldosterone stimulation. Three patients were subsequently treated with fludrocortisone (0.1 to 0.2 mg per day), with normalization of serum potassium levels. It is concluded that hyporeninemic hypoaldosteronism is responsible for hyperkalemia in some patients with AIDS and that treatment with fludrocortisone is effective in these cases.