In this paper we present data on the cognitive abilities analyzed by systematic, standardized psychometric testing in 18 fra(X) boys who participated in a multicenter, longitudinal study. In the majority of the patients no significant differences were found between verbal and performance intelligence. Higher performance IQ than verbal IQ was found primarily in higher functioning fra(X) males, indicating a possible effect of level of functioning on the direction of verbal-performance IQ differences. Subtest-analyses showed lowest performances in Number Concept and Arithmetic Skills, whereas better performances were reached in Object Assembly and Picture Completion. These data are discussed and compared with the results in previous studies reported up to now.