SummaryCholesterol is converted to cholic acid and chenodeoxycholic acid by a series of reactions involving modifications to the steroid nucleus and oxidation of the side chain. These reactions can be affected by a number of inborn errors of metabolism. When this happens unusual bile acids or bile alcohols are synthesized; these can be identified using gas chromatography‐mass spectrometry and fast atom bombardment mass spectrometry techniques. Two defects affecting the modifications to the steroid nucleus have been described; both present with cholestatic liver disease of neonatal onset. The better characterized of the two — 3β‐hydroxy‐Δ5‐C27‐steroid dehydrogenase deficiency — leads to excretion of 3β‐7α‐dihydroxy‐5‐cholenoic acid and 3β,7α,12α‐trihydroxy‐5‐cholenoic acid in the urine. The liver disease improves dramatically on treatment with chenodeoxycholic acid. Deficient activity of 3‐oxo‐Δ4‐steroid 5β‐reductase is thought to be the cause of familial liver disease in some infants who excrete 7α‐hydroxy‐3‐oxo‐4‐cholenoic acid and 7α,12α‐dihydroxy‐3‐oxo‐4‐cholenoic acid in the urine. However, diagnosis of this disorder is problematical; a similar pattern of metabolite excretion can occur as a result of liver damage caused by viruses or inborn errors of pathways unrelated to bile acid synthesis. Defective side chain oxidation in patients with cerebrotendinous xanthomatosis (CTX) leads to synthesis of bile alcohols such as 5β‐cholestane‐3α,7α,12α,25‐tetrol and 5β‐cholestane‐3α,7α,12α,23,25‐pentol. Patients with CTX do not have cholestatic liver disease. Their major problems (neurological disease, atherosclerosis and xanthomata) are caused by accumulation of cholestanol and cholesterol in the tissues. Bile acid precursors are probably diverted into synthesis of cholestanol. Chenodeoxycholic acid suppresses the production of abnormal metabolites from cholesterol (by inhibition of cholesterol 7α‐hydroxylase) and leads to improvement in the neurological disease. Defective side chain oxidation also occurs in peroxisomal disorders but this time it leads to accumulation of C27 bile acids such as 3α,7α,12α‐trihydroxy‐5β‐cholestanoic acid (trihydroxycoprostanic acid, THCA). This compound is readily detected in the bile and plasma of patients with defects of peroxisome biogenesis. In patients with defects of a single peroxisomalβ‐oxidation enzyme (the 3‐hydroxyacyl‐CoA component of the bifunctional protein or the thiolase), the major C27 bile acid in bile may be 3α,7α,12α,24‐tetrahydroxy‐5β‐cholestanoic acid (varanic acid). In addition to the above inborn errors, others which are less well characterized undoubtedly exist, as do defects of bile acid transport across membranes.