Recent cognitive-behavioral theories on obsessive-compulsive disorder (OCD) show that deliberate attempts to suppress intrusive and undesirable thoughts lie at the genesis of clinical obsessions. In this paper the results of an experimental study on the suppression of neutral and obsession-like thoughts in normal subjects are presented. Eighty-seven university students performed in three experimental periods: (1) base-line monitoring, (2) experimental instruction, and (3) monitoring. For each of these periods, the frequency of the occurrence of a "white bear" thought or a personally relevant intrusive thought was registered. Half of the subjects received instructions to suppress the target-thought in period 2, and the other half were instructed to only monitor the target-thought in each of the experimental periods. Several measures were also obtained before and after the experiment: annoyance caused by the intrusion, suppression effort, subjective success, and evaluative appraisals of the target-thought. The results showed neither immediate nor delayed frequency increases of the target thought. However, evidence was found that deliberate thought suppression efforts, regardless of their content, had greater negative consequences than did non suppression. These results are discussed in relation to the recent cognitive proposals about OCD.