It has been noted that obsessional patients appear to be equally sensitive to ideas regarding the possibility that they may cause harm by both their actions and by their failure to act (i.e., omission). This observation is discrepant with findings in non-clinical populations. The cognitive theory of obsessive-compulsive disorder suggests that it is the very occurrence of intrusive thoughts about potential harm that mediates this effect. In this study, 22 obsessional patients and 30 non-clinical participants were provided with details of ambiguous situations and either a negative or neutral intrusive thought pertaining to this situation. Behavioural and emotional responses to these situations were rated using self-reported measures. It was found that situations including an intrusive thought about harm were associated with higher intensity behavioural and emotional responses compared with the same situation when the intrusion was neutral. Obsessional participants scored higher overall; only on the rating of perceived responsibility was there an interaction between group and item type. These results are consistent with the idea that the occurrence of an intrusion about harm modifies both obsessional and non-clinical participants' reactions in ways that suggest obsessionality, and support cognitive theories that emphasize that obsessional experiences arise from normal processes.