To investigate the origin of the pulmonary systolic ejection innocent flow murmur (IFM), echocardiographic examinations were undertaken in 30 children with IFM and in a control group consisting of 28 healthy children without murmur. Compared to the controls, the diameters of the left ventricular outflow tract (LVOT) and aortic valve annulus and aortic valve area tended to be smaller, whereas stroke volume (SV) and cardiac output were slightly greater in children with IFM, but they were not statistically significant. Mean fractional shortening was significantly higher in children with IFM. Peak flow velocity of LVOT, right ventricular outflow tract, aorta, and pulmonary artery, mean velocity and velocity time integral of the aortic and pulmonary flow, and maximal acceleration of the aortic flow were all significantly higher in the IFM group compared to the controls. The variables of left-sided flow velocities in the same individuals with IFM were significantly higher compared to those derived from the right heart. The ratios of the SV to the LVOT diameter and to the aortic valve area were found to be significantly greater. It was concluded that IFM originates from higher blood flow velocities in the region of LVOT and aortic valve annulus, and that the increased flow velocity results from the larger SV passing through the relatively narrow LVOT and aortic valve in children with IFM.