Infantile Zellweger syndrome belongs to the group of peroxisomal disorders that lack peroxisomes. Both trihydroxycoprostanic acid (THCA), the precursor to cholic acid, and dihydroxycoprostanic acid (DHCA), the precursor to chenodeoxycholic acid, accumulate in this disease. In previous studies, we have shown that liver peroxisomes are required for the conversion of THCA into cholic acid bothin vitroandin vivoby measuring a defective conversion in infants with Zellweger syndrome. In our present study, the conversion of DHCA into chen-odeoxycholic acid has been measured in an infant with Zellweger syndrome to evaluate the importance of liver peroxisomes for the formation of chenodeoxycholic acid. Coprostanic acidemia was present from the second day of life with high levels of THCA and only trace amounts of DHCA. The conversion of i.v. administered I3H]DHCA into Chenodeoxycholic acid was only 7% compared with the 80% conversion in an analogous study in an adult. There was, however, a rapid incorporation of3H into biliary THCA and, after a lag phase, the 3H was incorporated into biliary cholic acid. After 72 h, 15% of I3H]DHCA was converted to cholic acid. The pool size of DHCA was 1.2 mg/m2and the pool size of both cholic acid and chenodeoxycholic acid was markedly reduced. The renal excretion of cholic acid was more efficient than that of the less polar chenodeoxycholic acid. More polar metabolites of DHCA and THCA are formed in alternative metabolic pathways facilitating renal excretion of these toxic intermediates. We conclude that liver peroxisomes are essential for a normal conversion of DHCA into chenodeoxycholic acid.