Disproportionate septal hypertrophy associated with erythroblastosis fetalis. 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. 