Three patients presented with severe congestive cardiomyopathy of unknown cause. All three had a profound depression of serum phosphorus levels resulting from the chronic ingestion of large quantities of a phosphorus-binding antacid. Results of physical examination and echocardiograms were consistent with cardiomegaly and severe myocardial dysfunction, and chest films showed enlargement of the cardiac silhouette with interstitial pulmonary edema. Serum phosphorus was restored to normal levels, and within 2 to 5 weeks the results of physical examination and echocardiogram of each patient returned to normal. We conclude that these patients had reversible hypophosphatemic cardiomyopathy and show the importance of inorganic phosphorus in myocardial metabolism and function. Serum phosphorus measurements should be a part of the routine evaluation of patients with congestive cardiomyopathy because, at least in some patients, hypophosphatemia appears to be a reversible cause of this disorder.