The authors review the literature on obsessive-compulsive disorder and present clinical vignettes to illustrate that delusions can arise in the course of this illness. These delusions do not signify a schizophrenic diagnosis but represent reactive affective or paranoid psychoses, which are generally transient. Using a phenomenologic analysis of 23 patients, the authors further argue that obsessive-compulsive disorder represents a psychopathological spectrum varying along a continuum of insight. Patients at the severe end of this spectrum are best described as having an "obsessive-compulsive psychosis." The authors discuss the implications of these considerations for DSM-III revisions.