OBJECTIVE: Macroorchidism is one of the most well-described clinical characteristics of men with fragile X syndrome, but little information has been available regarding macroorchidism in prepubertal boys with fragile X. The purpose of this study was to learn whether macroorchidism is a characteristic of fragile X syndrome in prepubertal boys. METHODS: Clinical assessments were performed on 73 boys, 2.2 to 10.2 years, in whom fragile X either had been diagnosed or who had been referred for fragile X evaluations because of developmental delays. As part of these assessments, we measured testicular volumes using Prader orchiometer beads to find out if macroorchidism occurs before the onset of puberty. RESULTS: From 2.2 through 7 years, boys with fragile X have significantly larger mean testicular volumes than boys without fragile X, but macroorchidism (> or = 4.0 mL or > or = 2 times normal) only occurred in 1 of 27 boys with fragile X. From 8 to 10 years, all our patients with fragile X had clinically enlarged testicles (> or = 3.75 mL), and 7 of 9 boys with fragile X had macroorchidism (> or = 4.0 mL). The testicular volumes of control subjects were all < or = 3.0 mL. CONCLUSIONS: These findings suggest that macroorchidism occurs in some prepubertal boys with fragile X, but in this series it did not occur until age 6 and usually did not occur until after age 8.