This study uses semi-structured interviews with obsessionals and nonclinical controls to investigate the frequency with which these individuals experience intrusions about possible harm and the frequency with which they then act to prevent that possible harm. The findings suggest that obsessionals do not generally experience more frequent intrusions about possible harm than do nonobsessionals, but that obsessionals more frequently experience intrusions in specific situations: obsession-relevant situations and situations they find most problematic. There was found to be a generalized difference between obsessionals and nonobsessionals in terms of frequency of actions taken to prevent potential harm following intrusions in situations that are obsession-relevant and obsession-irrelevant. The findings suggest that the occurrence of intrusions is just one factor influencing obsessional behaviour.