Schizophrenia is recognized by the presence of one or more clinical syndromes, but there is disagreement as to how far the boundaries of the concept should be extended. During the course of a World Health Organization study, using the Present State Examination and a computerized classification program, a nuclear schizophrenic syndrome was nearly always (95.1%) associated with a diagnosis of schizophrenic or paranoid psychosis. The only substantial exception was that 13 out of 79 patients diagnosed as manic were said to show the nuclear syndrome. The computer classification was concordant with a clinical diagnosis of schizophrenic or paranoid psychosis, manic psychosis, or depressive disorder, in 90% of cases. If appropriate precautions are taken, many of the sources of noncomparability in epidemiological, therapeutic, and prognostic studies can be brought under control.