The IQ of 52 girls with precocious puberty (mean age 9.5 +/- 2.8 years) was compared with that of 51 normal matched control subjects (mean age 9.7 +/- 2.8 years) and with that of eight girls with fast puberty (onset at normal age but accelerated advancement). Girls with precocious puberty had a significantly higher verbal IQ score than the control subjects but no difference was found in the performance score. The distribution of the verbal IQ score in the girls with precocious puberty was skewed toward the upper side of the theoretical distribution curve. The distribution was two or more times the expected theoretical percentile in the above average area (greater than 110, 56.9% v 25%), and five times more in the very superior area (greater than 130, 10.1% v 2.2%). The girls with fast puberty had the same behavior as the population with normal development. The results are interpreted as possible evidence of an effect of sex hormones on brain development, especially on the left hemisphere, during the prepubertal period.