Abstract This phonocardiographic study of 41 children and young adults with uncomplicated ostium secundum atrial septal defect has resulted in these findings: (1) No correlation could be found between mean splitting and the pulmonary-to-systemic flow ratio. (2) A positive correlation (r = 0.59, significant at α = 0.01) was observed between age and mean splitting. (3) An inverse correlation was noted between heart rate and splitting (r = −0.49, significant at α = 0.05). Possible mechanisms responsible for these relationships are discussed. A major clinical implication of these findings relates to the evaluation of pulmonary ejection murmurs in infants and young children. In these age groups, an atrial septal defect may exist in the presence of a normal or nearly normal pattern of splitting of the second heart sound.