Mental retardation (MR) is present in 2-3% of individuals in the general population, either as an isolated finding, or as part of an underlying disorder. It is the reason for a substantial part of referrals of patients and families to the pediatric genetic counseling units. MR may be genetically determined, or due to environmental (including perinatal) influences. Despite the thorough examination, physicians often are unable to define the etiology, in at least 30-50% of cases. This report presents the experience with 100 consecutive children with mental retardation, admitted to the Section of clinical genetics of the University pediatric hospital in Sofia over one year period. According to the routinely used classification, the patients' disease states have been subgrouped into genetic (35), multifactorial (3), environmental (3), and of unknown etiology (59). The research protocol included careful dysmorphologic, neurologic and developmental assessment, as well as cytogenetic, biochemical and molecular genetic testing. The rate of the diagnostic yield of the whole group was 41%. The discussion includes clinical examples and literature reports.