Birth weights of 147 cases of patients affected with sex chromosome anomalies were analyzed. In males with sex chromosome anomalies, there is a trend of decreasing birth weight with an increase in the number of X chromosomes. The addition of a Y chromosome is found not to be correlated with low birth weight. In the females, individuals affected with sex chromosome disorders are, on the average, born lighter than the female newborn controls, but there is no significant difference between the birth weight of each group of female patients with sex chromosome disorders. The evidence suggests that birth weight may be affected by aneuploidy involving the X and not the Y chromosome.