Left ventricular dimensions, wall thickness, wall stress, and indexes of afterload, preload, contractility, and early diastolic function, as well as regional wall motion, were determined by echocardiographic methods in patients with transposition of the great vessels after arterial switch operation and in age-matched normal controls. In patients evaluated early after surgery, body surface area-adjusted left ventricular dimensions were smaller, and wall thickness was increased compared with controls. Ventricular performance (fractional shortening) was normal in most patients but was abnormally low in 10%. Nevertheless, contractility was normal or augmented in all subjects, with a mean value higher than the control group. The reduction in systolic function was related to altered loading conditions with a combination of reduced afterload and preload combined with augmented contractility. These altered myocardial mechanics appeared to be secondary to routine therapy with digitalis and diuretics. Diastolic function was also normal with differences in the rate of peak filling and rate of wall thinning entirely attributable to differences in ventricular size and function, and normalized indexes of diastolic function were not different between patients and controls. Patients evaluated late after repair were found to have normal regional wall motion with no evidence to suggest regional dysfunction as might be seen with regional ischemia. Ventricular size, wall thickness, systolic function, afterload, preload, contractility, and early diastolic function were indistinguishable from control values. Indexes of diastolic function demonstrated the same relation to age, body surface area, and ventricular size and function in both patients and controls.(ABSTRACT TRUNCATED AT 250 WORDS)