Recent research has pointed to the importance of cognitive activity in interfering with sleep, and suggested a close relationship between worry and insomnia. To explore the relationship between worry and insomnia in more detail, a sample was studied in which worry and insomnia were combined in a 2 x 2 design. The content of sleep-interfering cognitions was explored both with a previously developed Sleep Disturbance Questionnaire and a newly developed checklist of the content of thoughts that arose if people could not sleep. Both supported the importance of a distinction between sleep-related and other thoughts. Whereas worried insomniacs show a broad range of sleep-interfering thoughts, the thoughts of non-worried insomniacs focused mainly on sleep itself.