BACKGROUND--The idea that blood passes through the left ventricular outflow tract with a flat velocity profile has recently been questioned. OBJECTIVE--To construct flow velocity profiles by magnetic resonance imaging over the whole cross sectional subaortic flow area in normal subjects. SUBJECTS--Nine people without heart disease aged 25 to 56. DESIGN--Cardiac gated cine magnetic resonance images were produced with a 1.0 T magnet perpendicular to the longitudinal axis of the left ventricular outflow tract 0.5 to 1.0 cm below the aortic annulus in mid-systole. Velocity was encoded every 30-40 ms throughout systole in the direction of flow perpendicular to the image plane. MAIN OUTCOME MEASURES--Systolic velocity-time curves reconstructed in nine different regions (area of each circle 0.2 cm2) of the subaortic flow area. The systolic peak velocity and the mean flow rate were taken as indices of regional flow. RESULTS--The spatial inhomogeneity of the peak velocity, calculated as the percentage ratio of the range of the regional measurements to their mean, averaged 18.2% (5.0%), and the inhomogeneity of the mean flow rate was 19.2% (3.5%). There were significant trends (P < 0.05) across the regional measurements towards highest peak velocities and mean systolic flow rate in the anteromedial sector of the subaortic flow area. The peak systolic velocity in the centre of the flow area averaged 98 (8) cm/s while the coincident spatial average was 94 (5) cm (P = 0.013). CONCLUSIONS--The distribution of systolic velocities across the left ventricular outflow tract is skewed towards fastest flow in the anteromedial sector of the flow area. The peak velocities measured in the centre of the flow area slightly overestimate the coincident spatial average velocities.