Background. Several studies suggest that many patients with schizophrenia have pre-morbid neurodevelopmental abnormalities. This study examines how behavioural abnormalities are associated with mild psychotic symptoms and later schizophrenic illness.

Methods. Maternal ratings on the Child Behavior Checklist (CBCL) of the early behaviour of 155 subjects were obtained at entry to the Edinburgh study of people at high risk of schizophrenia. These maternal ratings were compared in those with and without psychotic symptoms and used to predict the later onset of psychosis.

Results. The CBCL syndrome scores for the children prior to age 13 did not distinguish any of the study groups at entry to the study. In the ratings made for the subjects when aged from 13 to 16, delinquent behaviour and ‘other problems’ were weakly associated with these symptoms. However, with the exception of somatic symptoms and thought problems, the age 13–16 scales were significant predictors of later schizophrenic illness. This was true also for some of the ratings prior to age 13.

Conclusions. Various behaviours, in particular, withdrawn and delinquent–aggressive behaviour in adolescents at risk of schizophrenia may predict later onset of the illness. These behaviours, however, are far less predictive of isolated psychotic symptoms prior to psychosis onset.