The data presented are those from a two-year prospective study of 69 patients identified in the Nottingham field centre of the WHO Study of Determinants of Outcome of Severe Mental Disorders. Premorbid personality, childhood adjustment and adolescent adjustment were assessed at the patients' first presentation to psychiatric services with a psychotic illness. Ratings were made blind to diagnosis. Premorbid explosive and paranoid traits were commoner in patients with schizophrenia than in patients with other non-organic psychoses, and these traits were associated with later onset of schizophrenia. Premorbid schizoid traits were significantly commoner in patients with schizophrenia compared to patients with other psychoses, but only in those patients for whom a parent was the informant. Schizoid traits were no commoner in men with schizophrenia than in women, and were not associated with earlier age of onset. The findings suggest that premorbid personality, in men and women, may shape the expression of symptoms produced during an illness episode.