Objectives : In randomized cross over design , we compared the efficacy of sildenafil in relatively small doses with placebo in patients with left ventricular systolic dysfunction secondary to dilated or ischemic cardiomyopathy , The primary end point was change in exercise duration time on treadmill using naughton protocol , secondary end points werechange in the left ventricular dimensions , left ventricular systolic function and pulmonary artery systolic pressure as assessed by echocardiography .Background : The goals of heart failure treatment are (1) symptomatic relief of dyspnea and improvement in exercise capacity of the patient, (2) reduction of mortality and adverse outcome .Sildenafil was discovered in 1989 as a selective inhibitor of phosphodisterase enzyme (PDE) -5 which emerged as a safe and effective treatment of erectile dysfunction. It was approved by U.S food and drug administration as oral effective treatment of primary pulmonary hypertension .Mny studies had shown sildenafil to be beneficial in treatmentof heart failure due to left ventricular systolic dysfunction.Methods: After initial clinical evaluation, Doppler echocardiography and treadmill exercise test using Naughton protocol, 50 patients with left ventricular systolic dysfunction secondary to dilated or ischemiccardiomyopathy were randomized to placebo or sildenafil at dose of 25 mg twice daily.The evaluation was repeated after 6 weeks .After 2 weeks of wash out period , the patients were crossed over to alternate therapy.Final evaluation was performed after another 6 weeks of treatment.Results: 50 patients completed the study, in both groups together, the exercise duration time increased from 382.24 sec to 396.06 at the end of placebo stage which is statistically non significant and then increased to511.58 sec at the end of sildenail stage ((P =0.002).In both groups together the functional capacity increased from 4.34 METS to 4.41 METS at the end of placebo stage which is statistically non significant and then increased to 5.59 METS at the end of sildenail stage ((P =0.000).In both groups together there is no significant difference as regard the the left ventricular dimensions , left ventricular systolic function and pulmonary artery systolic pressure after sildenafil therapy compared with baseline and after placebo measuresConclusion: Sildenafil in relatively small doses significantly improves exercise duration time and functional capacity in patients with left ventricular systolic dysfunction secondary to dilated or ischemic cardiomyopathy without significant improvement of the left ventricular systolic function regardless of presence of significant pulmonary hypertension.