We have used blood oxygenation level dependent imaging with functional magnetic resonance imaging (fMRI) to investigate the visual cortex response to photic stimulation during and in the absence of continuous visual hallucinations. A patient with cortical Lewy body dementia who experienced persistent and vivid complex hallucinations underwent fMRI on and off treatment with risperidone. When he was not hallucinating, photic stimulation produced a normal bilateral activation in striate cortex. During hallucinations, very limited activation in striate cortex could be induced. We interpret this result as indicating that at least part of the activity in the brain responsible for the experience of visual hallucinations is located in the primary visual cortex.