Based on hypothesizing about the role of information processing, and in particular, underinclusive categorization in compulsive hoarding, this study examined categorization processes in people with clinically significant compulsive hoarding problems. Twenty-one participants with primary compulsive hoarding, 21 with OCD without hoarding, and 21 non-psychiatric controls completed three categorization tasks. Hoarding and OCD participants reported significantly more distress prior to each of the three tasks than did controls. On tasks sorting common household items, the groups did not differ on the number of piles created nor on the amount of time taken to sort. However, on a task sorting personally relevant items, hoarding participants took more time, created more piles, and reported more anxiety than non-psychiatric controls. Hoarders also took more time than the OCD group, and tended to create more piles. Hoarding severity was correlated with the number of piles created, but only when the objects were personally relevant. Results support under-inclusive categorizing for people with compulsive hoarding, but the effect was largely confined to objects of personal relevance.