Using data from the WHO International Pilot Study of Schizophrenia and from our own previously reported series of 120 patients receiving treatment for schizophrenia, we ascertained the degree of agreement between 13 different systems for diagnosing schizophrenia. We identified a triad of symptoms similar to that from the International Pilot Study: auditory hallucinations, passivity feelings, and disturbances of affect. This triad correlated very strongly with the diagnosis of schizophrenia as determined by the 13 diagnostic systems, as well as with response to fluphenazine in our series. The triad should serve as a core set of symptoms in the study of schizophrenic illness. It represents the leading phenomena in a group of patients having what is generally considered to be schizophrenia, although it does not provide a definitive diagnosis. Evidence is needed on its specificity.