Abstract: Nitric oxide is a free radical that is elevated in the plasma of patients with heart failure due to contractile dysfunction. This study examine the relation between plasma NO level and Left Ventricular (LV) diastolic function and its aetiology in heart failure patients in the pediatric age group. We performed echocardiographic Doppler studies in 47 patients (mean age of 6.16±2.8 years, 31 males and 16 females) with congestive heart failure. Left ventricular diastolic dysfunction was classified as either a restrictive (RFP) or non restrictive filling pattern (non-RFP). Same day venous total nitrite and nitrate levels were measured by colourimetric assay. Plasma NOx level was significantly higher in the studied patients than the control group (141±54 and 43±4 μmol/L, respectively, p<0.001). ROC curve found that the cut off point for plasma NOx level was 60 μmol/L to differentiate between normal children and patients with heart failure. Patients with RFP showed insignificantly higher levels of plasma NOx than the non-RFP patients. Only in muscular dystrophy patients, there were negative correlation between plasma NOx level and LV ejection fraction (r = -0.61, p = 0.06) and LV fractional shortening (r = -0.64 , p = 0.04). On correlating the plasma NOx levels to the severity of heart failure by multiple linear regression analysis, the pulmonary artery systolic pressure was the only variable independently associated with an elevated plasma NOx level (p = 0.05). Plasma NOx level is elevated in patients with isolated diastolic heart failure. In addition, in patients with LV systolic failure, the severity of LV diastolic dysfunction determines the amount of NO production.