Worry is crucial in the development and maintenance of anxiety disorders and has been associated with several other adverse health outcomes. Yet, little is known about the frequency and perseveration of worry in daily life, and its predictability by widely used trait questionnaires. In this study 432 students completed the Penn State Worry Questionnaire (PSWQ), the Worry Domains Questionnaire (WDQ) and the State-Trait Anxiety Inventory-Trait version (STAI-T) and kept a log of worry frequency and duration during six consecutive days and nights. The results showed that worry is a very common phenomenon that is predicted by the two trait worry questionnaires, independent of trait anxiety. The often clinically utilized PSWQ predicted worry duration better than the WDQ, and exclusively predicted night-time worry and several other indices of perseverative and potentially pathogenic worry. Although this study provides some support for the predictive validity of the PSWQ and the WDQ, these questionnaires did not account for the larger part of variance in daily worry. Future studies of worry and its associated perseverative processes should consider using momentary assessments.