Seven boys with an apparently X-linked syndrome of dilated cardiomyopathy, growth retardation, neutropenia, and persistently elevated urinary levels of 3-methylglutaconate, 3-methylglutarate, and 2-ethylhydracrylate were studied. The natural history of the disorder was characterized by severe or lethal cardiac disease and recurrent infections during infancy and early childhood but relative improvement in later childhood. The initial presentation of the syndrome varied from congenital dilated cardiomyopathy to infantile congestive heart failure to isolated neutropenia without clinical evidence of heart disease. The excretion of 3-methylglutaconate and 3-methylglutarate appeared to be independent of the metabolism of leucine, the presumed precursor of these organic acids in humans. Although the cause of the organic aciduria remains obscure, the constellation of biochemical and clinical abnormalities forms a distinct syndrome that may be a relatively common cause of dilated cardiomyopathy or neutropenia in boys during infancy and childhood.