Identical male twins and their brother, cholestatic from birth, with delta 4-3-oxosteroid 5 beta-reductase deficiency, were studied by serial liver biopsy. Spectrometry documented defective primary bile acid synthesis and markedly increased levels of atypical oxo and allo bile acids in urine and serum. Hepatocellular cholestasis and giant-cell transformation resolved in parallel with clinical and biochemical recovery during oral bile acid administration. In the twins, portal fibrosis stabilized at a mild level; they are well as 5-yr-olds at this writing. Follow-up biopsy in their brother at 8 mo was normal, and he is doing well at 3 yr of age. Hepatic ultrastructural alterations in all three were characterized by abnormalities of bile canaliculi including small bile plugs, diverticulae and latticelike elaborations of hepatocellular membranes adjacent to bile canaliculi that were shown to have resolved completely on subsequent biopsies. Eight additional cases have been detected on urine screening; only two of these patients have survived, on bile acid therapy. Early diagnosis and treatment improves the prognosis of this otherwise lethal inborn error of bile acid synthesis.