The trajectory of cognitive development in males with fragile X syndrome was identified in a cross-sectional study of 56 males and in a smaller, longitudinal study of 10 fragile X males. Results from both studies indicated steady cognitive growth until late childhood and early adolescence (10 to 15 years of age), at which point mental age plateaued and IQ declined. Males with higher initial IQ scores manifested more IQ decline than those with initial lower levels of intelligence. The trajectories of IQ differ from those in other etiological groups and mixed groups of retarded individuals. Results have direct implications for intervention strategies with fragile X males. Parents and teachers should be informed about the possibility of an early plateau in mental age, the impact this may have on the child's academic performance, and that the plateau and IQ decline may not be apparent in the child's adaptive behaviors.