Complete heart block was diagnosed prenatally in 21 fetuses. Associated structural cardiac defects were present in 18 fetuses, in particular complete atrioventricular canal with atrial isomerism (5 cases), and 'corrected' transposition of the great arteries (4 cases). Maternal systemic lupus erythematosus was proved in only one case. In 11 fetuses, intra-uterine congestive heart failure with the signs of non-immune hydrops fetalis occurred. In all 11 fetuses, the hydrops was associated with a cardiac defect, in particular complete atrioventricular canal with atrial isomerism in 5 cases. A review of the literature confirms that only the association of complete heart block and cardiac malformation can cause intra-uterine congestive heart failure, whereas in the case of fetal complete heart block without cardiac malformation or with prenatally hemodynamically insignificant cardiac malformation, congestive heart failure is rare. Only 30% of newborns with complete heart block have associated cardiac malformations. In our series, however, 86% of the fetuses with complete heart block had cardiac malformations. The most important reason for this percentage discrepancy is that almost all fetuses with associated severe cardiac defects, in particular atrioventricular canal defects, develop heart failure which frequently results in prenatal death. Thus, fetal deaths are not included in pediatric statistics. Nevertheless, fetuses with isolated complete heart block generally do not develop heart failure and in almost all of the cases are born alive.