This study aimed to determine the dimensionality of concepts of schizophrenia using 11 different diagnostic systems, and then to identify the nature of these dimensions by their relationship to a range of signs and symptoms. The sample consisted of 479 patients admitted with a first episode of functional psychosis. The underlying structure of the 11 diagnostic systems was best represented by an oblique 3-factor solution. Whereas the second and third factors could be meaningfully interpreted by their correlations with signs and symptoms, the first factor, anchored by 'modern' nosologies such as DSM-III-R, was more clearly specified by what it is not (the absence of affective symptoms) rather than by what it is (the presence of characteristic psychotic symptoms). A logistic regression of DSM-III-R diagnosis on to separate diagnostic components supports the contention that duration of illness and affective exclusion criteria discriminate the presence of DSM-III-R schizophrenia much better than the three characteristic psychotic symptom groupings.