This paper reviews the research on fragile X syndrome, the second most common cause of mental retardation related to chromosomal anomaly. It notes that far more males than females are affected by the fragile X syndrome, which typically results in craniofacial changes, delays in growth and development, speech/language difficulties, and cognitive deficits including mental retardation or learning disabilities. Topics reviewed include: diagnostic methods (direct DNA analysis is now preferred to cytogenetic analysis or DNA linkage studies); inheritance patterns (which explain the predominance of males); physical and cognitive characteristics; medical conditions associated with the condition (seizure disorders, motor coordination deficits); behavioral characteristics (hyperactivity, gaze avoidance); and speech/language characteristics (perseveration, cluttering, and echolalia). Other topics discussed are a possible relationship between autism and fragile X syndrome, whether language problems characteristic of the syndrome are due to qualitative or quantitative differences in language development, and the common presence of hypersensitivity. Interventions reviewed are organized into medical (especially drug therapy), and educational, which include classroom modifications, training in self-management skills, curricular modifications, and sensory integration therapy. (Contains 35 references.) (DB)