PURPOSE: Fragile X syndrome is an X-llinked genetic disorder and is characterized by mental retardation, learning disability, behavior disorder, and autism with typical elongated face, large ears, and macro-orchidism. Recent reports have focused attention on the EEG finding of this syndrome, which is a particular paroxysmal pattern during sleep (mono or diphasic centrotemporal spikes) and awake state (background slowing). In this study, we analyzed the clinical characteristics of fragile X syndrome patients and observed whether a particular EEG pattern is associated with this syndrome or not. METHODS: 7 cases of fragile X syndrome, diagnosed at Sowha Children's Hospital and Cha General Hospital from August 1993 to February 1995, were analyzed retrospectively in terms of typical phenotypes and clinical & EEG characteristics. The patients were diagnosed by Southern blotting and polymerase chain reaction (PCR) method. RESULTS: 1) The subjects were all male and the mean age was 5.8 years old (2Y-11Y). 2) Typical phenotype of long elongated face, macro-orchidism, large ears, and large head are noted in 2/3 of the subject. 3) Developmental delay, mental retardation, learning disability, attention deficit, hyperactivity, and autism are noted in 2/3 of the subject. 4) Seizure is noted in one case and EEG was performed in 6 cases, regardless of the presence of seizures. Abnormal findings including centrotemporal sharps and background slowing are noted in one case, each. 5) By molecular diagnostic methods including Southern blotting and PCR, 3 cases of affected male and 4 of normal transmitting male were diagnosed. CONCLUSIONS: 1) The typical phenotype of fragile X syndrome is long elongated face, macro-orchidism, large ears and large head. 2) The non-physical characteristics of fragile X syndrome are developmental delay, mental retardation, learning disability, attention deficit, hyperactivity, and autism. 3) The characteristic EEG findings of fragile X syndrome known by literature are noted in 2 among 6 cases, which means the specificity is high even though the sensitivity is low. This allows us to propose this EEG pattern as an important "marker" in the diagnosis of fragile X syndrome. However, the number of the subject is too small to conclude now. Further accumulation of cases is reguired.