We prospectively examined the usefulness of electrocardiography, chest radiography, and M-mode echocardiography in discriminating between the presence and absence of heart disease in 280 children older than one month and newly referred for evaluation of a heart murmur. After taking a history and performing a physical examination but before reviewing diagnostic tests, we categorized the children as having "no heart disease" (142), "possible heart disease" (34), or "definite heart disease" (104). Among the children initially thought to have no heart disease, the diagnosis was changed after a review of diagnostic tests in eight--three with mitral-valve prolapse, two with possible cardiomyopathy, and three with no heart disease on follow-up. Among those initially thought to have possible heart disease, the tests changed the diagnosis to definite heart disease in four, of whom only one had heart disease (mitral-valve prolapse) on follow-up. In no case did a review of tests change the diagnosis of definite heart disease. We conclude that the results of diagnostic tests are unlikely to change the clinical diagnosis of no heart disease or definite heart disease, when made by a qualified pediatric cardiologist in children newly referred for evaluation of a heart murmur.