Impairment in mental control is a primary complaint of many sufferers of obsessive-compulsive disorder (OCD). Most OCD sufferers work very hard to rid themselves of their obsessions, to little avail. Although active resistance is a defining feature of obsessions, it is typically not assessed in measures of OCD severity and little is known about the frequency of attempts at thought control or its impact on functioning while control strategies are engaged. In the present study, 37 individuals diagnosed with OCD kept a diary of their suppression attempts over a 3-day period, recording the circumstances under which the attempt at suppression occurred, the suppression strategy used, its outcome, and its impact on concentration, mood, peace of mind, and ability to proceed with planned activities. Results indicated that individuals with OCD engage in frequent, strenuous, time-consuming and ultimately unsuccessful attempts to control thoughts. Suppression was used as a means of avoiding the hassles associated with experiencing an obsession and with performing a compulsive ritual. Consistent with other research, suppression was also used as a means of neutralizing harm potentiated by the obsession. These findings suggest that thought suppression efforts and their impact may contribute significantly to the severity of impairment associated with OCD, and that it might be useful for clinical and research purposes to evaluate suppression as a severity indicator.