Plasma B-type natriuretic peptide (BNP) has been reported to be helpful in differentiating cardiac from pulmonary etiologies of dyspnea in adults. We investigated whether BNP concentration could be applied similarly in children with respiratory distress. BNP levels were measured using a rapid immunoassay in 49 infants and children presenting with acute respiratory distress. The patient's history, symptoms, physical exam, chest x-ray, and an echocardiogram were used to identify patients with congestive heart failure (CHF) from noncardiac causes of respiratory distress. Results are reported as mean+/-SD. Patients with CHF (n = 23) had BNP levels of 693.0+/-501.6 pg/ml, significantly higher than those of the group of patients with lung disease (n = 26), whose BNP was 45.2+/-64.0 pg/ml (p < 0.001). There was no significant difference in age between the two groups (29.7+/-59.3 vs 13.1+/-22.6 months; p = 0.12). A BNP level of 40 pg/ml was 84% accurate in differentiating CHF from pulmonary disease. Fifteen of 23 CHF patients had ventricular volume overload from left-to-right shunting congenital heart defects and 8/23 had left ventricular systolic dysfunction. Age-adjusted comparison of the two subgroups of CHF patients revealed that children with left ventricular systolic dysfunction had significantly higher mean BNP levels than those with left-to-right shunts (1181+/-487 vs 433+/-471 pg/ml, p = 0.0074). We conclude that BNP level is of value in differentiating cardiac from pulmonary causes of respiratory distress in children.