Two studies address the prevalence, concomitants, and appraisal of health-related intrusive thoughts. Eighty-three percent of adults (N = 658) in waiting rooms of two general hospitals reported at least one intrusive thought during the preceding month and 75.0% of patients and 55.5% of people accompanying them reported a health-related intrusive thought in the same period. The intrusions were associated with anxious and depressive symptoms. Health related intrusive thoughts were reported by 61% of a sample of university students (N = 608) and were the most frequent intrusive thought among 19.9% of the students. First, triggering stimuli reported by the subjects were significant predictors of thought frequency, worry, removal difficulty, and effort used in removing the thought. Second, appraisals of high probability were significant predictors of high frequency, worry, and especially difficulty in removing the thought. Finally, perceived responsibility and disapproval of the thought were also significant predictors of thought frequency, worry, removal difficulty, effort used in removing the thought, and guilt. These data support the position that cognitive appraisal of intrusive thoughts is closely linked to the subjective experience of the thought: more extreme appraisals were associated with more troublesome thoughts. The clinical implications of these studies are discussed in terms of current models of hypochondriasis and health anxiety.