We have identified 22 patients with the fragile X syndrome. The majority of these patients have come to our attention because several family members had significant mental retardation and/or dysmorphic facial features suggestive of fragile X syndrome. All patients underwent extensive examinations including determination of phenotypic abnormalities, measurement of testicular size and a thorough neurologic assessment. The Stanford-Binet intelligence test was used to assess their intellectual functioning. One hundred metaphase spreads of each patient were examined for fragile X chromosomes.The results of this investigation revealed that 82% of patients displayed brisk deep tendon reflexes and 41% had extensor plantar responses. Gaze eversion was observed in 77% of patients. Almost half of the subjects (47%) exhibited hyperactive behavior and had a stooped posture and gait. Incoordination was noted in 59% of the patients and blepharospasm was present in 38%. All patients were mentally retarded, ranging from mild to profound retardation. The subjects' language development was also retarded and they had marked difficulties with syntax, intonation and fluency. All patients had macro-orchidism. Their main dysmorphic features included coarse facies, large ears, prominent glabella, synophrys, large nose, prognathism, and abnormal dermatoglypics.