For genetic counseling in fragile X (fra(X)) families, it is important to know the diagnostic impact of the CGG repeat length in carriers of the fragile X mutation. We have analyzed the CGG repeat length in 106 males and 73 females who had inherited the maternal fra(X) mutation. The sensitivity, specificity, and predictive value of the CGG repeat analyses, as measured on Southern blots of PstI/EcoRI digests, were calculated. In males the sensitivity, specificity and negative predictive value were 99%, 100% and 94%; respectively. In females the specificity (60%) and, consequently, the positive predictive value (82%) was reduced. Therefore, it remains impossible to predict accurately whether a female fetus demonstrating a full mutation will be affected. On the other hand, the negative predictive value of the CGG test in females was 100%. We have no evidence, as yet, that for female carriers with the full mutation, cytogenetic analysis is of additional value to predict the mental status. However, we have observed one mentally retarded fra(X) case with extreme mosaicism in which a typical premutation fragment was the predominant DNA species. Therefore, until more data and better DNA tests are available, we would like to advocate additional cytogenetic investigation if in a fetus a CGG repeat length in the premutation range is found.