In 113 cases of fetal cardiac arrhythmia, i.e. 94 with supraventricular arrhythmia, 5 with atrioventricular block and 14 with ventricular arrhythmia, the clinical outcome was studied and compared with the general pregnant population. The arrhythmia group was afflicted with a significantly increased frequency of congenital malformations, 6.2% vs. 2.0%; fetal distress in labor, 20.4% vs. 13.5%; perinatal mortality, 3.5% vs. 0.7%; and neonatal mortality, 1.8% vs. 0.1%. In 4 cases, pharmacological cardiac treatment was needed in utero due to fetal heart failure. Fetuses with cardiac arrhythmia thus constitute an obstetric and pediatric high-risk group that should be subjected to an intensified supervision to detect fetal heart failure or fetal distress. When indicated, these complications can be treated in utero.