We prospectively examined the developmental trajectories of receptive and expressive communication skills of 39 young males, 20 to 86 months of age, with fragile X syndrome. Eight showed features characteristic of autism. Children were tested one to three times using a standardized language test. They showed marked delays in language development, but substantial individual variability. Participants acquired expressive language skills more slowly than receptive language over time, gaining receptive language at about half the rate expected for typically developing children and expressive language at one third the rate. Both cognitive skills and autistic characteristics of the young males with fragile X syndrome related to receptive and expressive communication development, but neither predicted the discrepancies between expressive and receptive language acquisition over time.