Fetal behavior was studied in 17 cases of fetal abnormalities including nonimmune hydrops fetalis and in 11 cases of intrauterine growth retardation (IUGR) in comparison to 30 healthy fetuses. Breathing movements, rapid eye movements and trunk movements were observed ultrasonically and analyzed using a computer-assisted system. The cases with central nervous system anomalies showed a markedly different behavioral pattern. In the fetal abnormalities without chromosomal anomalies, the cases without major behavioral disorders were cured by neonatal treatment. IUGR cases with decreased trunk, breathing and REM period were finally diagnosed as having fetal distress by fetal heart rate monitoring. The findings on fetal behavior hint at the possibility of diagnosing abnormalities in neural development as well as in well-being.