Temperament has been demonstrated clinically to be linked to mental disorders. We aimed to determine the possible role of temperament in mental disorders in a national epidemiologic study. A nationally representative sample of adults (n=1320) was administered the Lebanese-Arabic version of the Temperament Evaluation of the Memphis, Pisa, Paris and San Diego Autoquestionnaire (TEMPS-A), and the Arabic CIDI 3.0, as part of the LEBANON study. The association among temperaments and DSM-IV mood, anxiety, and impulse control disorders was assessed. The anxious temperament was shown to be a robust predictor of most disorders, especially within the anxiety and depressive clusters. The hyperthymic temperament had a uniquely protective effect on most mental disorders, with the exception of separation anxiety, bipolar, substance abuse and impulse control disorders. These effects were moderated by age and education. Temperaments, previously largely neglected in epidemiologic studies, could play a major role in the origin of mental disorders.