Effective management of Hoarding Disorder (HD) must begin with assessment of the severity of hoarding symptoms and functional impairment. We sought to validate the UCLA Hoarding Severity Scale (UHSS), a semi-structured, clinician-administered rating scale that measures the severity of both the core symptoms of HD and the associated features of indecisiveness, perfectionism, task prolongation, and procrastination, which are significantly associated with the diagnosis and impairment of HD.