Intelligence levels and intelligence profiles were investigated in 52 members of a large family with the fragile X syndrome. The mental abilities were evaluated by the three Wechsler intelligence tests (WAIS, WISC-R, and WPPSI). Chromosomal and psychological data were then compared. In 22 non-retarded fra(X) negative family members, a mean IQ of 102 was found (males 97, females 106). Eleven non-retarded carrier females had IQs between 70 and 110 (mean 86.8), which is 20 points below the mean of normal women (alpha = 0.01). One non-retarded male with 6% fra(X) positive cells had an IQ of 98. His intelligence profile closely resembled the profile in the non-retarded female carriers. The highest IQ in the group of retarded males was 31. The mentally retarded females scored IQs between 26 and 41. In male and female patients verbal intelligence substantially exceeded performance abilities. There was a considerable gap between the highest IQ in the group of retarded females and the lowest IQ in the group of non-retarded carriers (41 and 71 respectively) and a considerable overlap was found between the IQ levels of the non-retarded carriers and normal women.