Residual function of the left ventricle was assessed in 25 patients with mitral stenosis and a normal left ventriculogram. The post-extrasystolic beat (R2) in sinus rhythm (nine patients) and the first beat after an early beat (R2) in atrial fibrillation (16 patients) were analysed angiocardiographically. Five subjects with a normal heart (controls) were also studied. The results are expressed as percentage changes in left ventricular contractility from the beat preceding the extra beat (R1) to the beat R2. In the control group the mean changes from R1 to R2 were: end diastolic volume +68.3% (increase), end systolic volume -21.7% (decrease), ejection fraction +36.2%, mean systolic ejection rate +22.1%, and mean velocity of circumferential fibre shortening +31%. A significant increase in proportional systolic shortening of all left ventricular axes was found in R2 compared with R1. In five patients with sinus rhythm and nine with atrial fibrillation the results fell within the normal range. In the remaining patients the beat R2 indicated signs of poor left ventricular function. The mean changes from R1 to R2 in the patients with sinus rhythm and those with atrial fibrillation were respectively: end diastolic volume +47.8% and +36.6%, end systolic volume +20% and +27%, ejection fraction +12.5% and +6.2%, mean systolic ejection rate -23.3% and -30.2%, and mean velocity of circumferential fibre shortening -25.5% and -39.2%. The increase in the left ventricular axial systolic shortening was not significant. Thus analysing a post-extrasystolic beat in sinus rhythm of the beat following an early beat with a long diastole in atrial fibrillation is a valuable method of determining the residual function in patients with mitral stenosis who have a normal left ventriculogram in basic rhythm.