The 3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4)isomerase (3beta-HSD) isoenzymes are responsible for the oxidation and isomerization of Delta(5)-3beta-hydroxysteroid precursors into Delta(4)-ketosteroids, thus catalyzing an essential step in the formation of all classes of active steroid hormones. The 3beta-HSD gene family should have evolved to facilitate differential patterns of tissue- and cell-specific expression and regulation involving multiple signal transduction pathways, which are activated by several growth factors, steroids, and cytokines. In humans, there are two 3beta-HSD isoenzymes, which were chronologically designated type I and II encoded by HSD3B1 and HSD3B2 gene, respectively. HSD3B1 gene encodes the almost exclusive 3beta-HSD isoenzyme expressed in the placenta and peripheral tissues, whereas HSD3B2 gene encodes the predominant 3beta-HSD isoenzyme expressed in the adrenal gland, ovary, and testis and its deficiency is responsible for a rare form of congenital adrenal hyperplasia causing various degrees of salt-wasting in both sexes and incomplete masculinization of the external genitalia in genetic males. Although an elevated ratio of Delta(5)-Delta(4)-steroids was considered to be the best biological parameter for the diagnosis of this autosomal recessive disorder, the most accurate criteria now appears to be the plasma levels of 17-OH-pregnenolone greater than 100 nmol/L following ACTH stimulation. To date a total of 34 mutations (including 5 frameshift, 4 nonsense, 1 in-frame deletion, 1 splicing, and 23 missense mutations) have been identified in the HSD3B2 gene in 56 individuals from 44 families suffering from classical 3beta-HSD deficiency. In almost all the cases, the functional characterization of HSD3B2 mutations has provided a molecular explanation for the heterogeneous clinical presentation of this disorder. Indeed these experiments confirm that no functional 3betaHSD type II isoenzyme is expressed in the adrenals and gonads of the patients suffering from a severe salt-wasting form, whereas the non-salt-losing form results from specific missense mutation(s) in the HSD3B2 gene, which causes an incomplete loss of enzymatic activity thus leaving sufficient enzymatic activity to prevent salt wasting. Moreover, various mutations appear to have a drastic effect upon stability of the protein, therefore providing molecular evidence of a new mechanism involved in classical 3beta-HSD deficiency. Thus, the elucidation of the molecular basis of 3beta-HSD deficiency has highlighted the fact that mutations in the HSD3B2 gene can result in a wide spectrum of molecular repercussions, which are associated with the different phenotypic manifestations of classical 3beta-HSD deficiency and also provide valuable information concerning the structure-function relationships of the 3beta-HSD superfamily.