Objective: To determine quantitative changes in ventricular chamber volumes and left ventricular mass after orthotopic cardiac transplantation. Material and Methods: Right ventricular and left ventricular chamber volumes and left ventricular muscle mass were quantified by electron beam computed tomography in 10 patients at 1 month and 12 months after orthotopic cardiac transplantation. Results: During the study period, the mean right ventricular end-diastolic volumes increased from 117 ±26.1 cc to 143 ±25.3 cc (P<0.005), and the mean left ventricular end-diastolic volumes increased from 90 ±20.6 cc to 117 ±27.3 cc (P<0.001). Left ventricular stroke volume increased from 63 ±16.5 cc at 1 month to 78 ±19.8 cc by 12 months (P<0.005). These changes were associated with a concurrent reduction in global left ventricular muscle mass from 168 ±25.2 g to 145 ±16.3 g (P<0.01). The left ventricular enddiastolic volume/mass ratio, an index of wall tension, was abnormally low at 1 month but was in the normal range by 12 months (P<0.0005). Both left ventricular and right ventricular ejection fractions were normal at all times and not significantly changed between the 1-month and 12-month studies. Conclusion: Significant right and left ventricular remodeling occurs between 1 month and 12 months after orthotopic cardiac transplantation, with progressive cavity dilatation but reduction in global left ventricular muscle mass. These adjustments, especially in the left ventricle, tend to reduce the volume/ mass ratio by 1 year and are compatible with normalization of ventricular wall tension between the early and late scan dates. Routine clinical measurements of ejection fraction alone mask the almost parallel increases in stroke volume and end-diastolic volume during the first 12 months after orthotopic cardiac transplantation. © 1996; Mayo Foundation for Medical Education and Research.