To explore the function of StAR in a system that can be experimentally manipulated, and to develop a mouse model for the human disorder lipoid congenital adrenal hyperplasia (lipoid CAH), we used targeted gene disruption to produce a mouse line deficient in StAR protein. Initially, StAR knockout mice were indistinguishable from wildtype littermates, except that all had female external genitalia. After birth, they showed signs of either respiratory distress or volume depletion and eventually died. Hormone assays confirmed severe defects in adrenal steroids, whereas hormones constituting the gonadal axis did not differ significantly from levels in wildtype littermates. Histologically, the adrenal cortex of StAR knockout mice contained florid lipid deposits, as visualized with oil red O stain. Lesser lipid deposits were observed in the steroidogenic compartment of the testis and none in the ovary. The sex-specific differences in gonadal involvement provide evidence for a two-stage model of the pathogenesis of StAR deficiency, with trophic hormone stimulation causing progressive accumulation of lipids within the steroidogenic cells which ultimately kills them. These StAR knockout mice provide a novel system in which to study StAR's essential roles in adrenocortical and gonadal steroidogenesis.