The most common enzymatic defect of steroid synthesis is deficiency of the adrenal steroid 21-hydroxylase. Inhibition of the formation of cortisol results in an increased pituitary release of ACTH which in turn drives the adrenal cortex to overproduce androgens. This hormonal setting affects the development of genetic females by misdirecting the differentiation of external genitalia towards the male type. Since the isolation of the gene encoding 21-hydroxylase enzyme in 1984, gene deletions, large gene conversions, and microconversions have been reported to be responsible for the disease. In this paper, we report a study of this genetic defect in 22 families with one or more affected offspring diagnosed as having the classical form of congenital adrenal hyperplasia. The DNA from 30 patients was analyzed with three restriction enzymes. Hybridization with a 21-hydroxylase cDNA probe and the 5 'end of a C4 genomic probe disclosed gene deletion in 7.3% (3/41) of the disease-related chromosomes. The rate of large gene conversion was 17.1%(7/41), and no abnormality in the hybridization pattern was observed in 75.6% (31/41) of the disease alleles. Densitometry of the autoradiographs was used to determine the ratio of the copy-number of the 21-hydroxylase gene (CYP21B) to the copy-number of its pseudogene (CYP21A). Differences in phenotype, the low frequency of gene deletion, and the high frequency of gene conversion compared with other studies in different populations indicated that 21-hydroxylase deficiency in the Brazilian population may involve different molecular mutations.291113