Studied 2 factors hypothesized as relevant to obsessional problems because of the way they influence decisions whether or not to act to prevent harm. These are (1) the way in which intrusive thoughts increase the internal awareness of harm, and confront the person with the possibility of taking action to prevent such harm and (2) the extent to which there is some obvious external factor that increases awareness of the possibility of preventing harm. 42 obsessional patients, 25 anxious and 53 non-clinical controls completed a scale measuring these factors across a wide range of situations. Both obsessionals and nonobsessionals were more likely to report acting to prevent harm when awareness of it is prompted by an intrusion than when it is not. Ss in all groups acted more obsessionally when a scenario is described in ways that suggest that harm may be by commission than omission. When scenarios about which each S is most disturbed were analyzed, anxious and non-clinical Ss continued to differentially rate omission and commission situations, while obsessional patients did not. Thus, obsessionals are more sensitive to omission than are nonobsessionals when considering scenarios about which they are concerned; this sensitivity influences the decision whether to act to prevent harm.