Patients with obsessive-compulsive disorder (OCD) not only respond to obsessions with perseverative checking, but also engage in more general checking, irrespective of their obsessive concerns. This study investigated whether general checking is specific to OCD and exacerbated when only mild uncertainty is induced. Thirty-one patients with OCD, 26 anxiety- and 31 healthy controls performed a visual search task with eye-tracking and indicated in 50 search displays whether a target was “present” or “absent”. Target-present trials were unambiguous, whereas target-absent trials induced mild uncertainty, because participants had to rely on not overlooking the target. Checking behavior was measured by assessing search time and the number of fixations, measured with an eye-tracker. Results showed that in both target-present and target-absent trials patients with OCD searched longer and made more fixations than healthy and anxiety controls. However, the difference in checking behavior between patients with OCD and the control groups was larger in target-absent trials (where mild uncertainty was induced). Anxiety and healthy controls did not differ in checking behavior. Thus, mild uncertainty appears to specifically promote checking in patients with OCD, which has implications for treatment.