Findings from a prospective longitudinal study of 46 boys with fragile X syndrome between the ages of 24 and 72 months were reported. Hierarchical linear modeling was used to construct and evaluate overall developmental trajectories and scores in five domains: Cognition, Communication, Adaptive, Motor, and Personal-Social. The children varied widely, with significant differences across individuals in both mean rate and level of performance. Overall development was significantly delayed, with a slope of .48— approximately half the rate expected for typically developing children. No differences were found in rates of growth across the five domains. Significant differences were found, however, in mean levels of performance. At every age tested, Motor and Adaptive scores were higher than Communication and Cognitive.