The aim of this study is to establish the prevalence of fragile X syndrome among Estonian mentally retarded and also among the entire children's population born during the years 1984-2005. The study group consisted of 516 patients (448 boys and 68 girls) who were screened for full mutations in the FMR1 gene during the period 1997-2006. Fourteen boys (2.7%) were found with full mutations of the total mentally retarded individuals tested (3.1% of mentally retarded boys); the full mutation was not detected among girls. The live-birth prevalence of full mutation among boys was 1:13 947. The overall live-birth prevalence of fragile X syndrome was 1:27 115. It was found that the prevalence of fragile X syndrome among mentally retarded individuals in Estonia was the same as in previous studies, but the live-birth prevalence of fragile X syndrome among boys was significantly lower.