OBJECTIVE: We sought to compare our 5-year program of fragile X screening of high-risk gravid women with our program of fragile X testing of affected individuals (probands). STUDY DESIGN: All women referred to the prenatal genetics clinic from 1994 to 1998 who had a family history of unspecified mental retardation or learning or behavioral disorders (known fragile X families excluded) were offered fragile X screening. Results were compared with those of probands with the same diagnoses who underwent fragile X testing during the same time period. RESULTS: We counseled 12,349 prenatal patients from 1994-1998, of whom 263 (2.1%) had a positive family history and underwent fragile X screening. No mutations or premutations were identified. In contrast, 31 (1.9%) of 1637 affected probands who underwent fragile X testing during the same time period had positive results, which was a significant difference (0/263 vs 31/1637; P < .05). CONCLUSIONS: Testing the affected proband is superior to screening the pregnant relative of the proband for identification of families at risk for fragile X syndrome.