Aims: To examine temporomandibular disorder (TMD) patients' illness beliefs and self-efficacy in relation to bruxism, and to examine whether these beliefs are related to the severity of patients' self-perceived bruxing behavior. Methods: A total of 504 TMD patients (75% women; mean age ± SD: 40.7 ± 14.6 years), referred to the TMD Clinic of the Academic Centre for Dentistry Amsterdam, completed a battery of questionnaires, of which one inquired about the frequency of oral parafunctional behaviors, including bruxism (clenching and grinding). Patients' illness beliefs were assessed with a question about the perceived causal relationship between bruxism and TMD pain; patients' self-efficacy was assessed with questions about the general possibility of reducing oral parafunctional behaviors and patients' own appraisal of their capability to accomplish this. Results: Sleep bruxism or awake bruxism was attributed by 66.7% and 53.8% of the patients, respectively, as a cause of TMD pain; 89.9% believed that oral parafunctions could be reduced, and 92.5% believed themselves capable of doing so. The higher a patient's bruxism frequency, the more bruxism was believed to be the cause of TMD pain (Spearman's rho 0.77 and 0.71, P .001) and the more pessimistic the self-efficacy beliefs were about the reducibility of oral parafunctions (Kruskal-Wallis χ2 = 19.91, df = 2, P .001; and Kruskal-Wallis χ2 = 7.15, df = 2, P = .028). Conclusion: Most TMD patients believe in the harmfulness of bruxism and the possibility of reducing this behavior. Bruxism frequency is associated with illness beliefs and self-efficacy. Keywords: bruxism, illness beliefs, oral parafunctions, self-efficacy, temporomandibular disorders