Left ventricular (LV) outflow obstruction may result from retaining the anterior mitral leaflet when a mitral prosthesis is inserted in the mitral anulus. We retrospectively reviewed the chocardiograms (two-dimensional Doppler and Doppler color flow imaging, or transesophageal with color flow imaging) obtained in seven patients with preoperative mitral regurgitation who had a prosthesis implanted with the native mitral leaflets left intact. Systolic anterior motion of the native anterior mitral leaflet, as seen in dynamic LV outflow tract obstruction, was observed in six of seven patients. LV fractional shortening preoperatively was ≤0.25 in all (mean 0.20 ± 0.04) and did not significantly (p = ns) Increase postoperatively (mean 0.27 ± 0.12). Color flow imaging revealed disturbed systolic flow in the LV outflow tract in five patients, and all had systolic anterior motion of the native anterior mitral leaflet. Continuous wave Doppler detected significant systolic LV outflow tract jets in five patients averaging 4.1 ± 0.9 m/sec. Mitral prosthetic function was normal (pressure half-time of 81 ± 25 msec and mean gradient of 7 ± 3 mm Hg ± SD) in five patients. Clinical follow-up revealed that all had died, six of them within 2 months of their operation. Thus systolic anterior motion of the native anterior mitral leaflet occurs commonly after prosthetic mitral valve insertion with the native leaflets left intact. Continuous wave Doppler often demonstrates increased systolic LV outflow tract velocities consistent with dynamic LV outflow obstruction. Therefore the presence of LV outflow obstruction in patients after prosthetic mitral valve insertion with retained native leaflets may result in an adverse postoperative course and requires close follow-up.