We present the case of a 23-year-old Vietnamese male with a 2-year history of a psychotic illness marked by prominent negative symptoms, fatuousness and disturbed behavior. Neuroimaging revealed a prominent vascular flow void affecting the middle and anterior cerebral arteries, with associated increased collateral supply to the frontal cortex, consistent with Moyamoya disease. Neurological examination was unremarkable; however, neuropsychological assessment revealed significant executive dysfunction, including stimulus-driven behavior. Whilst the diagnosis of schizophrenia and Moyamoya disease may be coincidental, an interaction between the 2 diseases may have led to some of the atypical features of this case, including prominent executive dysfunction and marked sensitivity to psychotropic medication. We discuss the nature of possible interactions between the 2 conditions. This case also highlights the importance of re-evaluating patients with atypical or treatment-resistant psychoses for cerebral pathology. (JINS, 2003, 9, 806–810.)