Rumination on symptoms of mental disorders is involved in the onset and maintenance of these symptoms across a range of mental disorders. The purpose of the study was to investigate whether rumination on unwanted intrusive thoughts (UITs) has an immediate causal effect on discomfort, urge to do something about the UITs (i.e., to neutralize) and frequency of the UITs, as well as on depressed mood. A UIT was activated by asking nonclinical participants to write down a sentence stating that they wished a loved one would die in a horrible car accident. During the experimental manipulation, they were instructed to ruminate on their UIT, to ruminate on negative mood, or to distract themselves by thinking about everyday objects and situations. Individuals who had previously ruminated on the UIT had an attenuated reduction of the urge to neutralize compared to individuals who had previously engaged in rumination on negative mood or those who were distracted. Results indicate that thinking repetitively about a UIT prevents decay of the urge to engage in behaviors to undo it but does not influence discomfort or depressed mood associated with it. This phenomenon may be involved in the maintenance of UITs, for example, in obsessive-compulsive disorder.