The Associação dos Pais e Amigos dos Excepcionais (APAE) is an institution for mentally retarded patients located at Caxias do Sul in the south of Brazil. A genetic diagnostic survey of 202 individuals from this institution is presented. The patients had a male:female ratio of 1.3:1 and their ages varied from 1 month to 47 years with a mean of 5.5 years. Using personal and family data, careful clinical examination and laboratory investigation, the authors established a definitive diagnosis in 132 patients (65.34%). A constitutional disorder was present in 111 patients (54.95%). Down's syndrome patients represented 32.15%, while 1.98% had other chromosomal anomalies. In 25 patients (12.37%) a disorder of Mendelian inheritance was diagnosed. In 8 patients (3.96%) a multiple congenital anomalies/mental retardation (MCA/MR) syndrome was recorded. Eight patients (3.96%) had a central nervous system (CNS) malformation. An acquired condition was observed in 21 patients (10.39%), including pre- or post-natal infections. In the remaining 70 patients (34.65%) a conclusive diagnosis was not possible.