Background: Hoarding Disorder (HD) is currently under consideration for inclusion as a distinct disorder in DSM-5 (1). Few studies have examined comorbidity patterns in people who hoard, and the ones that have suffer from serious methodological shortcomings including drawing from populations already diagnosed with obsessive compulsive disorder (OCD), using outdated definitions of hoarding, and relying on inadequate assessments of hoarding. The present study is the first large-scale study of comorbidity in a sample of people meeting recently proposed criteria for hoarding disorder (1) and relying on validated assessment procedures. Methods: We compared psychiatric comorbidity in a large HD sample (n = 217) to 96 participants meeting criteria for OCD without HD. Results: High comorbidity rates were observed for major depressive disorder (MDD) as well as acquisition-related impulse control disorders (compulsive buying, kleptomania, and acquiring free things). Fewer than 20% of HD participants met criteria for OCD, and the rate of OCD in HD was higher for men than women. Rates of MDD and acquisition-related impulse control disorders were higher among HD than OCD participants. No specific anxiety disorder was more frequent in HD, but social phobia was more frequent among men with HD than among men with OCD. Inattentive ADHD was diagnosed in 28% of HD participants and was significantly more frequent than among OCD participants (3%). Conclusions: These findings form important base rates for developing research and treatments for hoarding disorder. Depression and Anxiety, 2011. © 2011 Wiley-Liss, Inc.