A clinical population of 39 families affected by fragile X syndrome, a sex-linked form of mental retardation, is described. Physical aspects, including such common features as prominent jaw and simple ears, are noted along with psychological aspects such as different mean IQs among various age groups. Findings of intellectual evaluation did not show any predictive relationship between number of fragile sites observed in chromosomal analysis and level of intellect. Levels of retardation extended from the mild to the profound range. It is noted that folic acid therapy may benefit the syndrome, although it is an extremely controversial topic. Tables and figures illustrate the data. (CL)