Synopsis The study reports a comparison of 10 definitions of schizophrenia (Feighner's, Taylor's, Schneider's, Langfeldt's, Spitzer's, Carpenter's, Astrachan's, 2 from Forrest & Hay and Catego) in respect of their reliability, concordance and prediction of outcome. Some (including Feighner's) proved too strict, and others (including Schneider's) were weak predictors of clinical and social state. Four definitions (Spitzer's, Langfeldt's, Carpenter's and Catego) emerged as relatively effective and these also had a somewhat more satisfactory mutual concordance.