[Severe mental retardation in an urban child population].

The author studied the prevalence of severe mental retardation in Vilnyus and considered the status of its etiological and clinical differentiation. A total of 224 cases of oligophrenia were identified among children aged 4-13 years which constituted a populational rate of 0.31% in the above age group. A marked predominance of males among the cases with undifferentiated forms was ascertained, which is probably due to the accumulation in this group of X-chromosome-linked mental retardation. Cytogenetic examination of 20 suspected cases revealed the syndrome of a fragile X chromosome in 4 patients.