Worry has been defined as a primarily cognitive process closely related to fear or anxiety. This definition is widely accepted by clinicians and researchers investigating worry and is often provided to research participants, especially to children. The present study aimed to empirically test the proposition that both adults and children conceptualise worry as a cognitive process, and that they differentiate it from fear on the basis of worry's relatively stronger association with thinking. Groups of 45 adults and 70 children completed three versions of a worry list questionnaire, reporting on the extent to which they worry about, are afraid of, and think about a list of negative outcomes. Regression analyses showed that in adults, thinking about a negative outcome had a significant unique association with worrying. In children, however, worrying was more strongly associated with fear ratings, especially in the case of physical outcomes. The data suggest that adults and children report on different concepts when they rate their worries via questionnaires. This difference may reflect developmental changes in the worry process itself, or developmental changes in the way individuals interpret and respond to worry list questionnaires. Follow-up studies using alternative methodologies are now needed to further clarify the results.