Forty-four fetuses with pericardial effusions have been identified by ultrasonographic examinations. The clinical histories and courses of these patients were reviewed. At least eight different clinical features accompanied and were probably responsible for the pericardial effusions. The most common cause of a fetal pericardial effusion was heart failure (13 fetuses). Fetal renal cystic dysplasia with oligohydramnios and other anomalies was present in six fetuses. Four of this group had microscopic evidence of pericarditis on postmortem examination. Fetal pericardial effusions are always a manifestation of another disease process often presenting as fetal hydrops. Some pericardial effusions are transient. The etiologic origin of fetal pericardial effusions differs from that in the child or adult.