Expression of the fragile site Xq27.3 was investigated in 82 heterozygotes (58 normal and 24 mentally impaired) diagnosed by DNA analysis. EcoRI and EagI DNA digests were probed with StB12.3. This allowed the detection of the expansion of the CGG repeat of the FMR1 gene and the methylation pattern of the adjacent CpG island. Heterozygotes with delta < or = 400 bp (52/82) were all mentally normal and manifested fra(X) in less than 3% of the cells or did not express it. Unmethylated mutant alleles were always observed. About two thirds of females with delta > 500 bp (21/30) showed fra(X) frequencies above 3% (3 normal and 18 mentally impaired). Lower frequencies of fra(X) or negative results were observed in the remaining 9 females (3 normal and 6 affected). The large mutant alleles were always methylated. Therefore, while delta < or = 400 bp is always associated with negative or low expression of fra(X), larger expansions are not present exclusively in heterozygotes with high frequencies of fra(X). In 25 of 30 heterozygotes with delta > 500 bp, active and inactive normal alleles were observed. Three fra(X)-negative or low manifesting heterozygotes showed completely skewed X-inactivation, with the normal allele either active or inactive. Two females with high frequencies of fra(X) always had the normal allele inactivated. Densitometry studies showed no difference in the inactivation of the normal allele between heterozygotes who manifested fra(X) or not. Thus fra(X) expression does not seem to be influenced by X-inactivation.