The frequency of diurnal clenching and/or grinding and nail-biting habits was assessed in patients affected by temporomandibular disorders (TMDs) and in healthy controls in order to investigate the possible association between these oral parafunctions and different diagnostic subgroups of TMDs. The case group included 557 patients (127 men, mean age +/- SD = 34.5 +/- 15.4 years; 430 women, mean age +/- SD = 32.9 +/- 14.1 years) affected by myofascial pain or disc displacement or arthralgia/arthritis/arthrosis. The control group included 111 healthy subjects (55 men, mean age +/- SD = 37 +/- 15.2 years; 56 women, mean age +/- SD = 38.2 +/- 13.8 years). Multinomial logistic regression analysis was used to assess the association between oral parafunctions and TMDs, after adjusting for age and gender. Daytime clenching/grinding was a significant risk factor for myofascial pain (odds ratio (OR) = 4.9, 95% confidence interval (CI): 3.0-7.8) and for disc displacement (OR = 2.5, 95% CI: 1.4-4.3), nail biting was not associated to any of the subgroups investigated. Female gender was a significant risk factor for myofascial pain (OR = 3.8; 95% CI: 2.4-6.1), whereas the risk factor for developing disc displacement decreased with ageing. No association was found between gender, age and arthralgia/arthritis/arthrosis.