Autism-spectrum disorder is increasingly recognised, with recent studies estimating that 1% of children in South London are affected. However, the biology of comorbid mental health problems in people with autism-spectrum disorder is poorly understood. Aims To investigate the brain anatomy of people with autism-spectrum disorder with and without psychosis. Method We used in vivo magnetic resonance imaging and compared 30 adults with autism-spectrum disorder (14 with a history of psychosis) and

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... healthy controls. Results Compared with controls both autism-spectrum disorder groups had significantly less grey matter bilaterally in the temporal lobes and the cerebellum. In contrast, they had increased grey matter in striatal regions. However, those with psychosis also had a significant reduction in grey matter content of frontal and occipital regions. Contrary to our expectation, within autism-spectrum disorder, comparisons revealed that psychosis was associated with a reduction in grey matter of the right insular cortex and bilaterally in the cerebellum extending into the fusiform gyrus and the lingual gyrus. Conclusions The presence of neurodevelopmental abnormalities normally associated with autism-spectrum disorder might represent an alternative 'entry-point' into a final common pathway of psychosis.