OBJECTIVE--The origin of innocent systolic murmurs is uncertain. This study was designed to evaluate the relative contribution in the same subjects of the main factors implicated in the genesis of vibratory innocent systolic murmurs--that is, the presence of left ventricular bands and increased left ventricular flow indices--and to examine evidence for turbulence in relation to these findings. PATIENTS--40 children with innocent systolic murmurs and 40 paired controls without murmurs were matched for age and body surface area. DESIGN--These subjects were examined prospectively by cross sectional echocardiography with pulsed and continuous wave Doppler and colour flow mapping. SETTING--A tertiary referral centre for paediatric cardiology. RESULTS--Left ventricular bands were significantly more prevalent in patients with innocent murmurs (70%) than in controls (48%) but only if apical bands were included (p = 0.039). Cases had significantly higher aortic peak flow velocities (p less than 0.001), flow integrals (p = 0.001), and stroke volumes (p = 0.014), and significantly lower heart rates (p = 0.033) than controls. There were no significant differences between cases and controls in mean aortic diameter, cardiac output, or ejection fraction. There was no evidence of turbulence in the left ventricle or outflow tract in any individual but curvilinear periodic vibrations and audible "whoops" were detected in the left ventricle by pulsed wave Doppler echocardiography. These were significantly more common in cases than controls (p less than 0.01) but were not related to the presence of bands. CONCLUSIONS--Innocent systolic murmurs were unrelated to aortic size but they were strongly associated with higher aortic flow volume and velocity, with relative bradycardia, and with evidence of vibratory phenomena in the left ventricle. The role of left ventricular bands remains uncertain.