The effects of combination therapy with chenodeoxycholic acid (CDCA) and simvastatin on serum cholestanol, low-density lipoprotein (LDL) cholesterol, and lathosterol levels were investigated in seven adult patients with cerebrotendinous xanthomatosis (CTX) who were on long-term treatment with CDCA. The patients were treated with a combination of CDCA 750 mg daily and an increasing dose of simvastatin from 10 mg to 40 mg daily for a period of 6 months. We found a significant effect of this combination therapy compared with CDCA alone in terms of decreasing the serum cholestanol and LDL cholesterol levels, particularly with a daily dose of 40 mg simvastatin. The mean cholestanol level decreased from 9.27 micromol/L (baseline) to 6.69 micromol/L (40 mg simvastatin), while the mean LDL cholesterol level decreased from 5.08 mmol/L (baseline) to 3.04 mmol/L (40 mg simvastatin). No side effects were reported, and there were no effects on the clinical condition, cerebral magnetic resonance imaging (MRI), visual evoked potentials, and electroencephalographic features. We conclude that a combination of 750 mg CDCA and 40 mg simvastatin daily is effective to further reduce serum cholestanol, LDL cholesterol, and lathosterol in adult CTX patients treated with long-term CDCA. Whether this combination treatment will be effective for the long-term prevention of neurological deterioration and atherosclerosis remains to be established.