Niemann-Pick C1 (NPC1) disease is a rare, neurodegenerative lysosomal cholesterol storage disorder, typified byprogressive cognitiveand motor function impairment.Affected individuals usuallysuccumbtothe disease in adolescence. 2-Hydroxypropyl-b-cyclodextrin (HP-β-CD) has emerged as a promising intervention that reduces lipid storage and prolongs survival inNPC1disease animal models.Abarrier to the development of HP-β-CD and other treatments forNPCdisease has been the lack of validated biochemical measures to evaluate efficacy. Here we explored whether cholesterol homeostatic responses resulting from HP-β-CD-mediated redistribution of sequestered lysosomal cholesterol could provide biomarkers to monitor treatment. Upon direct CNS delivery of HP-β-CD, we found increases in plasma 24(S)-HC in two independent NPC1 disease animal models, findings that were confirmed in humanNPC1 subjects receiving HP-β-CD. Since circulating 24(S)-HCis almost exclusively CNS-derived, the increase in plasma 24(S)-HC provides a peripheral, non-invasive measure of the CNS effect of HP-β-CD. Our findings suggest that plasma 24(S)-HC, along with the other cholesterol-derived markers examined in this study, can serve as biomarkers that will accelerate development of therapeutics for NPC1 disease.