Standards (95th, 50th, and 5th percentiles) in fragile X syndrome for weight, height, head circumference, ear length, and testicular volume are reported. For comparison with fragile X syndrome standardized curves, normal control data from the literature were similarly plotted and curves produced. These standards reflect the physical parameters that are frequently abnormal in males with the fragile X syndrome and should be useful in the medical management of patients with this syndrome. The standards may also be used to help identify those individuals, particularly the younger males, for chromosome studies to confirm the clinical impression of the fragile X syndrome.