P450c17 is a single microsomal enzyme that catalyzes two distinct steroid biosynthetic activities: 17 alpha-hydroxylase and 17,20 lyase. Human beings have only one gene that encodes only one form of P450c17. Three clinical observations indicated that these were independently regulated activities. First, several cases of isolated 17,20 lyase deficiency were reported, in which 17 alpha-hydroxylase activity was spared. Second, most adrenal steroidogenesis in children stops after 17 alpha-hydroxylation, thus permitting the synthesis of cortisol, whereas most gonadal steroidogenesis proceeds to C19 sex steroids as a result of both activities. Third, the 17,20 lyase activity of the human adrenal is developmentally activated during adrenarche. To catalyze these two activities, P450c17 must receive reducing equivalents from electron donors (redox partners). Previous observations showed that the molar ratio of P450 oxidoreductase to P450c17 was 3-fold higher in the testis than in the adrenal, and that increasing the molar ratio of the redox partner to P450c17 would increase the ratio of 17,20 lyase activity to 17 alpha-hydroxylase. We have recently shown that P450c17 must be phosphorylated on serine and threonine residues by a cAMP-dependent protein kinase to acquire 17,20 lyase activity. We have also recently found two cases of isolated 17,20 lyase deficiency that have mutations of residues in the proposed redox partner binding site. Together, these studies suggest a unified view of the regulation of 17,20 lyase activity. The ratio of 17,20 lyase to 17 alpha-hydroxylase activity of P450c17 is regulated by the availability of reducing equivalents flowing to the enzyme. This can be increased by increasing the molar concentration of electron-donating redox partners, such as P450 oxidoreductase or possibly cytochrome b5, as appears to be the case in the gonads. Alternatively, the affinity of P450c17 for redox partners may be selectively increased by Ser/Thr phosphorylation, or selectively decreased by certain mutations in the redox partner binding site, in either case altering an electrostatic interaction between P450c17 and the redox partner. This model is consistent with all present observations about the biochemistry, genetics, enzymology, and clinical phenomenology of P450c17.