Pathological doubt, a prominent feature of obsessive-compulsive disorder (OCD), may be related to difficulty tolerating ambiguous or uncertain situations. This is thought to be particularly true of those patients with checking compulsions. Intolerance of uncertainty (IU) has been studied extensively within the domains of worry and generalized anxiety; however, it has received relatively little empirical attention in OCD patients. We administered the Intolerance of Uncertainty Scale [Personality and Individual Differences 17 (1994) 791] to 55 clinic patients with OCD, 43 of whom had checking compulsions, and 14 nonanxious controls. OC checkers showed greater IU than did OC noncheckers and NACs. The latter two groups did not differ from each other. Furthermore, both repeating and checking rituals were associated with IU. Pathological doubt may be understood not only in terms of knowledge-based constructs, but also patients' emotional reaction to feelings of uncertainty. We discuss the implications for increasing tolerance via cognitive-behavioral therapy.