Abstract: This study was carried out to compare echocardiographic findings of children taking tacrolimus and cyclosporin A (CsA) after orthotopic liver transplantation (OLT). Echocardiograms of 19 children were reviewed during hospitalizations after OLT, and echocardiograms were performed on 23 children who returned to the clinic for a routine follow‐up visit after OLT. Measurements were made of the left ventricle (LV) end‐diastolic dimension, and of the thickness of the LV free wall (LVFW) and the inter‐ventricular septum (IVS). From these measurements, the LV mass was calculated. LV outflow gradient was measured by using Doppler interrogation. Comparisons were made between patients on CsA and patients on tacrolimus. Children with hypertrophic cardiomyopathy (HCM) were identified. Two patients from the in‐patient tacrolimus group were found to have HCM. These two patients had asymmetric septal hypertrophy with dynamic LV outflow obstruction and were successfully treated with propranolol, with or without discontinuing tacrolimus. In the out‐patient studies, there was no difference in LVFW and IVS thickness, or LV mass index, between children on CsA and children on tacrolimus. Hence, tacrolimus is associated with the development of HCM in children. The effect of tacrolimus on HCM development may be acute and temporary. More data are needed to determine the incidence of HCM in children on tacrolimus therapy and to establish guidelines for clinicians who follow‐up these children.