We retrospectively reviewed clinical and echocardiographic data on 10 newborns with erythroblastosis fetalis who were admitted to our nurseries between 1984 and 1988 and who required a double-volume exchange transfusion and neonatal intensive care. Echocardiograms were performed in the first 48 hours of life. In 5 patients, disproportionate septal hypertrophy was demonstrated; 1 additional patient had biventricular hypertrophy with a thickened septum but not disproportionate septal hypertrophy. The mean septal: left ventricular free-wall ratio for the group (n = 10) was 1.37. No correlation was apparent between the occurrence of disproportionate septal hypertrophy and newborn glucose, bilirubin, or hematocrit values. When analyzed separately, the 4 patients who did not receive intrauterine blood transfusions had a ratio of 1.73 +/- 0.21 (mean +/- SEM); this was significantly greater than the ratio in the 6 patients who were transfused in utero (1.13 +/- 0.24). In patients who underwent transfusions, there was no correlation between the number of transfusions and the septal:left ventricular ratio. This study reports a significant but previously unrecognized cardiac hypertrophy with disproportionate septal hypertrophy in patients with erythroblastosis fetalis. Our data suggest a sparing effect of intrauterine fetal transfusions. The mechanism by which these transfusions may affect the hypertrophic development of the myocardium remains to be determined.