In this study, the authors compared the phonological accuracy and patterns of sound change of boys with fragile X syndrome, boys with Down syndrome, and typically developing mental-age-matched boys. Participants were 50 boys with fragile X syndrome, ages 3 to 14 years; 32 boys with Down syndrome, ages 4 to 13 years; and 33 typically developing boys, ages 2 to 6 years, who were matched for nonverbal mental age to both the boys with fragile X syndrome and the boys with Down syndrome. All participants were administered a standardized articulation test, and their sound accuracy, phonological process, and proportion of whole-word proximity scores were analyzed. Although boys with fragile X syndrome were delayed in their speech development, they did not differ from the typically developing, mental-age-matched boys in the percentage of correct early-,middle-,and late-developing consonants; phonological processes; or whole-word proximity scores. Furthermore, boys with fragile X syndrome had fewer errors on early-,middle-,and late-developing consonants; fewer syllable structure processes; and higher whole-word proximity scores than did boys with Down syndrome. Boys with Down syndrome also were delayed in their speech development, yet their phonological inventories, occurrences of phonological processes, and proportion of whole-word proximity scores indicated greater delays in their phonological development than the younger, typically developing boys. These results suggest that males with fragile X syndrome display phonological characteristics in isolated words similar to younger, typically developing children, whereas males with Down syndrome show greater delays as well as some developmental differences compared with both the males with fragile X syndrome and typically developing males. (Contains 3 tables and 3 figures.)