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Accumulating evidence suggests that schizophrenia is associated with altered cerebral laterality secondary to a deviation from normal brain development. A number of findings suggest that age of onset of psychosis and gender may have a significant bearing on the nature and extent of the deviation. In order to examine this, early onset patients (12-19 years of age) were compared to healthy controls and later onset patients in a series of studies using standard neuropsychological techniques, experimental divided visual field (DVF) measures and magnetic resonance imaging (MRI). Specific attention was directed to examining the influence of sex and age of onset on hemispheric specialisation. In the neuropsychological studies, early onset patients (n=35) demonstrated significant impairment of intellectual functioning relative to normal adolescents (n=35) but no significant VIQ-PIQ discrepancy. Earlier age of onset was significantly correlated with reduced VIQ and FSIQ. Early onset patients showed significant reduction in hand skill, increased incidence of non-right eye preference and crossed hand-eye dominance. In addition, patients demonstrated reduced right ear advantage (REA) in dichotic listening and inability to modulate ear advantage by directing attention. In the DVF experiments, early onset patients (n=20) demonstrated normal lateralisation in phonological word recognition but sexually dimorphic anomalies in lexico-semantic processing relative to normal controls (n=20). Males showed impairment in imageable word recognition whereas females were more impaired in emotional word recognition. In both cases, the observed anomalies implicated a disturbance in the semantic network subserved by left hemisphere ventromedial and superior temporal heteromodal cortex. In MRI investigations, early onset patients (n=33) had smaller cerebral hemispheres and larger lateral ventricles than controls (n=32). Male patients showed reduction of leftward asymmetry in temporal lobe volume and female patients showed reversal of rightward asymmetry. Significant correlations were found between left ventricular brain ratio and reaction time to phonological word processing. Together, the combined results indicate that early onset schizophrenia is associated with a significant but selective alteration of cerebral laterality, that age of onset is likely to be a determinant of this alteration and that, to some extent, these changes are mediated by gender. The results are discussed within the context of neurodevelopmental aetiology.