• The ability of six different operational definitions of schizophrenia to identify prospectively patients whose eventual prognosis would be poor was studied using data from a six-year follow-up of a series of 134 patients with functional psychoses. All six definitions were more successful at predicting a poor symptomatic outcome than a poor social outcome. Spitzer's Research Diagnostic Criteria, Carpenter's flexible criteria, and Langfeldt's criteria predicted a poor outcome as well as the original clinical diagnoses and were considerably better than the New Haven criteria, Schneider's first rank symptoms, or the computer program Catego.