Left ventricular (LV) function was studied from end-systolic stress-shortening relations in 13 patients with mitral valve prolapse (MVP). Studies were made noninvasively using M-mode echocardiography with blood pressure measurements on the right arm from a Baumanometer cuff. Fifteen age and sex-matched normal subjects formed the control group. LV end-diastolic dimension was normal in MVP, while shortening fraction (p < 0.05) and velocity of circumferential fiber shortening (mean Vcf) in the short axis of the ventricle were increased (p < 0.05). Ventricular afterload, as measured by meridional end-systolic stress was reduced in MVP (p < 0.01) but the slope of the stress-shortening curve was not different form normal. The decrease in end-systolic stress was related to the severity of prolapse (r = 0.63, p < 0.05). The study implied that mid- or basal LV contractility was normal in MVP. The increased shortening fraction of the LV minor axis was in keeping with geometrical changes and a reduced ventricular afterload.