Fifty cases of nonimmunologic hydrops fetalis found in Japanese infants are reported. Nonimmunologic hydrops fetalis is associated with various pathological conditions, twin transfusion syndrome including acardiac monsters, fetal heart diseases, congenital cystic adenomatoid malformation, pulmonary sequestration, pulmonary lymphangiectasia, intrauterine infections such as cytomegalovirus infection and neonatal hepatitis, congenital neuroblastoma, Kasabach-Merritt syndrome, cystic hygroma, and chromosomal aberrations. The mechanism of hydrops fetalis found in these conditions is discussed from various viewpoints. Despite a careful examination, no causative conditions were found in 14 cases. The placenta showed a proliferation of Hofbauer cells that were strongly positive for immunoreactive alpha 1-antichymotrypsin and there were other common findings such as edema of terminal villi and fibrin thrombi.