A four-year retrospective survey of individuals referred for fragile X
testing to South East Thames Regional Genetics Service was carried ou
t to determine the accuracy of clinical diagnosis of fragile X syndrom
e among routine referrals for cytogenetic confirmation. 680 individual
s from 565 pedigrees were tested for fragile X. Five affected males we
re 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 explan
ation for the low diagnosis rate is failure of referral of affected ma
les.