It has been hypothesized that personally relevant negative intrusive thoughts may be the direct precursors of obsessional thinking. Efforts made to suppress or neutralize are said to be involved in the transition between 'normal' and 'abnormal' intrusive thoughts. In order to test this hypothesis, naturally occurring intrusive thoughts were identified in non-clinical subjects, some of whom were asked to suppress these thoughts during an experimental period. The occurrence and characteristics of the target intrusive thoughts during suppression and a subsequent non-suppression period were assessed. The results indicate that (i) suppression can result in increased intrusion; and (ii) that distraction can play an important moderating role. It is suggested that, in suppression experiments, the effects of self-monitoring per se may have been underestimated. Motivated and intrusion-focussed monitoring of consciousness may be as important as suppression, given that obsessional patients tend to be constantly vigilant for the occurrence of obsessional thinking.