We performed this study to assess whether low dose spironolactone could be administered in hemodialysis (HD) patients with moderate to severe heart failure to improve cardiovascular function and reduce hospitalization without inducing hyperkalemia. We enrolled 16 chronic HD patients with moderate to severe heart failure and left ventricle ejection fraction :5 45%. In a double blinded randomized placebo controlled study, one group of 8 patients received 25 mg of spironolactone after each dialysis session within six months, and the rest received a placebo. Echocardiography was performed on all the patients to assess ejection fraction and left ventricular mass during 12 hours after completion of hemodialysis at the beginning and the end of study. Serum potassium was measured predialysis every 4 weeks. The mean ejection fraction increased significantly more in spironolactone group during the study period than in the placebo group (6.2 +/- 1.64 vs. 0.83 +/- 4.9, P= 0.046). The mean left ventricular mass decreased in the spironolactone group, but increased significantly in the placebo group during the period (-8.4 +/- 4.72 vs. 3 +/- 7.97. 95%, P= 0.021). The incidence of hyperkalemia was not significantly increased in the study or controlled groups. In conclusion, we found in this study that administration of spironolactone in chronic HD patients with moderate to severe heart failure substantially improved their cardiac function and decreases left ventricular mass without development of significant hyperkalemia.