ObjectivesSeveral studies have shown that perseverative, worrisome thoughts are prospectively related to subjective health complaints (SHC) and that a short worry postponement intervention can decrease these complaints. As SHC and worry are prevalent and costly, we tested whether the intervention can be offered online to reduce these complaints in the general population. DesignA randomized parallel-group trial was conducted with self-selected participants from the general population. MethodsVia the research website, 996 participants were instructed to register their worrying for 6 consecutive days. The intervention group was instructed to postpone worry to a special 30-min period in the early evening. The Subjective Health Complaints inventory, as administered before and after the intervention, and daily worry frequency and duration were considered the primary outcomes. ResultsThree hundred and sixty-one participants completed the study. Contrary to our expectation, the registration group (n=188) did not differ from the intervention group (n=163) in SHC (p2=.000, CI [0.000-0.003]), or in worry frequency or duration. Nevertheless, the different worry parameters were moderately related to SHC (r between .238 and .340, p.001). ConclusionsIn contrast to previous studies using pen-and-pencil versions of the worry postponement intervention, this study suggests that a direct online implementation was not effective in reducing SHC and worry. Overall, participants had high trait worry levels and reported difficulty with postponing worrying. Reducing SHC and worries via the Internet might require more elaborate interventions that better incorporate the advantages of delivering interventions online. What is already known on this subject? The perseverative cognition hypothesis argues that perseverative cognition, such as worry and rumination, acts as a mediator by which psychosocial stress may produce negative health effects. Prior research has indeed shown that worry and subjective health complaints (SHC) are prospectively related, but causality studies - that is, showing that changes in worry induce changes in health outcomes - are scarce and have mainly been conducted in young samples. These studies showed that reducing worry, using a worry postponement intervention, can reduce daily worrying and SHC. What does this study add? Trait and daily worrying are associated with SHC. An online worry postponement intervention is ineffective in reducing worry and SHC. Paper-and-pencil interventions cannot directly be used as online interventions.