The cognitive-behavioural model of health anxiety hypothesizes that the degree of health threat experienced by an individual is a function of the perceived: (1) likelihood of illness; (2) awfulness of illness; (3) difficulty coping with illness; and (4) inadequacy of medical services. While research has examined cognitions in health anxiety, it is not known whether these cognitions predict health anxiety in individuals who do or do not report medical conditions and whether these cognitions are uniquely related to health anxiety. After developing the Health Cognitions Questionnaire to assess these specific cognitions, we examined the extent to which the cognitions predicted health anxiety and poor response to reassurance in a healthy community sample (n = 273) and a sample who self-reported various medical conditions (n = 208). Supporting the cognitive-behavioural model, these cognitions predicted health anxiety and poor response to reassurance in both samples, with some differences observed between those who did or did not report medical conditions. The cognitions were uniquely related to health anxiety even after controlling for depression and general anxiety. Clinical and theoretical implications are discussed. Overall, the Health Cognitions Questionnaire has potential to facilitate further research on the development, maintenance, and treatment of health anxiety.