A four-year retrospective survey of individuals referred for fragile X testing to South East Thames Regional Genetics Service was carried out to determine the accuracy of clinical diagnosis of fragile X syndrome among routine referrals for cytogenetic confirmation. 680 individuals from 565 pedigrees were tested for fragile X. Five affected males were identified in previously unknown families and 17 new pedigrees were diagnosed. Using the accepted prevalence data, a total of 80 affected males would have been expected in this period. The most likely explanation for the low diagnosis rate is failure of referral of affected males.