Cerebrotendinous xanthomatosis (CTX) is an inherited metabolic disease due to biallelic pathogenic variants in CYP27A1. We report a newly diagnosed patient and the outcome of the chenodeoxycholic acid (CDCA) treatment. This is a 36-year-old male with progressive lower limb spasticity, learning difficulties, and early cataracts. He was diagnosed by targeted next generation sequencing panel for hereditary spastic paraparesis (c.379C>T; p.Arg127Trp and c.1072C>T; p.Gln358* in CYP27A1) with CTX at the age of 33 years. Brain and spine magnetic resonance imaging (MRI) revealed increased T2 signal intensity in the dentate nuclei and bilateral posterolateral spinal cord. Baseline plasma 7a-hydroxy-4-cholesten-3-one (> 5; ref. range < 0.300 nmol/mL) and 7a,12a dihydroxycholest-4-en-3-one (> 5; ref. range < 0.100 nmol/mL) were markedly elevated. Baseline full-scale IQ was 69. The CDCA treatment (750 mg/day) was started at the age of 34 years. Plasma 7a-hydroxy-4-cholesten-3-one level was normalized; plasma 7a,12a dihydroxycholest-4-en-3-one level was markedly improved (0.755 nmol/mL; ref. range < 0.1) and full-scale IQ improved to 83 at two years of the CDCA treatment. This patient highlights improvements in neurocognitive functions despite late diagnosis and late initiation of treatment and exemplifies the importance of diagnosing a treatable disease at any age to improve neurocognitive outcomes.