The diagnostic status of paranoia (renamed simple delusional disorder [SDD]) has been debated since the time of Kraepelin. Three main views have emerged that consider paranoia (1) a subtype of schizophrenia, (2) a subtype of affective illness, or (3) a distinct nosologic entity. The literature pertinent to the diagnostic validity of a SDD is examined. Although the methodology of a number of the studies reviewed would not be considered rigorous by today's standards, certain tentative conclusions can nonetheless be reached. The available data do not suggest that SDD is a subtype of affective illness. The evidence reviewed also does not strongly support the hypothesis that SDD is a subtype of schizophrenia. Although some of the data is consistent with the view that SDD is a "mild" form of schizophrenia, the bulk of the evidence suggests that SDD and schizophrenia are distinct syndromes.