The assessment of left ventricular function in pediatric age generally has been made by measuring the extent and velocity of fiber shortening at the endocardium and relating these parameters to systolic wall stress. It has been suggested from animal and human studies that this "endocardial method" can result in an overestimation of myocardial function when healthy hearts and those with hypertrophy are compared. Thus an assessment of left ventricular pump function and contractility that takes into account the epicardial migration of the midwall circumferential fibers during systole (midwall analysis) is warranted. Although the normal range for midwall indexes of left ventricular mechanics has been established in adult subjects, up to now it has not been studied in pediatric subjects. To establish normal values for left ventricular midwall mechanics, 70 healthy children ranging in age from 3 days to 18 years were evaluated with 2-dimensional and M-mode echocardiography. Midwall fractional shortening (FSmw), rate-corrected mean velocity of circumferential fiber shortening (VCFcmw), and the relation between these indexes and left ventricular end-systolic wall stress (ESS) were calculated separately for the children younger than 2 (group 1, 20 subjects) and older than 2 (group 2, 50 subjects) years of age. Group 1 had significantly higher FSmw and VCFcmw compared with group 2. An inverse linear relation between FSmw and ESS and between VCFcmw and ESS was found in both age groups. The y-intercept was higher in group 1, and the slope of the mean regression line was steeper than in group 2 for both the relationships, suggesting an age-dependent midwall left ventricular pump function and contractility. These normative data can be used to assess the left ventricular midwall mechanics in pediatric patients with pressure or volume overload.