Objective: We aimed to determine whether N-terminal pro-brain natriuretic peptide can differentiate between cardiac and pulmonary aetiologies of dyspnoea, if N-terminal pro-brain natriuretic peptide can be used for evaluating the effect of treatment in cardiac failure, and for predicting severe pulmonary diseases that are complicated by cardiac failure. Methods: In all, 76 children with dyspnoea were enrolled; 41 of them suffered cardiac failure - 25 caused by cardiac disease, 16 caused by pulmonary disease - and 35 had dyspnoea due to pulmonary disease. The control group consisted of 32 children. We calculated Ross scores, analysed N-terminal pro-brain natriuretic peptide levels, and evaluated left ventricular systolic functions by echocardiography. Results: N-terminal pro-brain natriuretic peptide levels were significantly higher in children with cardiac failure than in those with pulmonary disease and in controls (medians 7321, 241, 87 71 picograms per millilitre, respectively), were higher in children with cardiac failure due to pulmonary disease than in those with only pulmonary disease (medians 2728, 241 picograms per millilitre, respectively), and were higher in children who died from cardiac failure than in survivors (p<0.05). After treatment of cardiac failure, N-terminal pro-brain natriuretic peptide levels decreased significantly (p<0.001). The cut-off level of N-terminal pro-brain natriuretic peptide for differentiating cardiacfailure from pulmonary disease was 726.8 picograms per millilitre, sensitivity 100%, specificity 94 3%. Conclusions: N-terminal pro-brain natriuretic peptide levels can differentiate dyspnoea clue to cardiac failure from pulmonary diseases. It can also be used to monitor the effects of treatment of cardiac failure and to estimate the prognosis, as well as to predict pulmonary diseases that are complicated with cardiac failure.