A 13-item checklist that combines physical and behavioral traits typical of fragile X [fra(X)] syndrome was evaluated prospectively in the screening of 107 males with mental retardation or severe learning disabilities. The checklist was completed before we obtained cytogenetic results. Fifteen males were fra(X)-positive and the manifestations that differentiated fra(X)-positive and fra(X)-negative patients included perseverative speech, large or prominent ears, large testicles, and tactile defensiveness. The combination of physical and behavioral traits is helpful in suggesting the diagnosis and identifying high-risk patients. A total score of 16 or higher had a significant yield of fra(X)-positive patients (greater than or equal to 45%).