The study of the developmental outcome of neonatal polycythemia was performed on 47 polycythemic and 21 controlled infants who were born at the same period of time. It was found that at the age of 1 1/2 to 2 years the number of infants with abnormal DQ was higher in the group of total polycythemic infants (47%) and in the group of asymptomatic polycythemic infants (45%) than that of the control groups (19% and 5.6% of the groups including twin sibs and excluding twin sibs respectively). There was no difference in the developmental test between the symptomatic and asymptomatic patients. In asymptomatic infants the benefit of partial plasma exchange transfusion on developmental outcome was not found and only low birthweight and small for gestational age infants are the risk factors for poor developmental outcome.