A 64-year-old right-handed man acutely developed elementary visual hallucinations (monochromatic, moving geometrical figures), visual illusions (distortion of the right side of faces) with achromatopsia and blurred vision restricted to the left visual hemi-field. CT of the brain before and after administration of contrast medium and a repeat examination 2 months later showed no abnormalities, while brain mapping (power analysis of EEG) demonstrated theta wave slowing of the curve over the posterior part of the right hemisphere. 99mTC HMPAO SPECT of the brain, however, demonstrated an area of definite focal hypoperfusion in the right occipito-temporal region. Echo-Doppler-duplex and continuous wave examination of the cervical arterial blood vessels disclosed bilateral discrete atheromatous plaques that did not affect the blood flow. Transoesophageal echocardiography demonstrated slight mitral valve insufficiency. Cerebral angiography showed an occlusion of the right posterior cerebral artery. After the visual hallucinations had subsided, SPECT showed partial normalization of the right occipito-temporal perfusion. In the absence of CT evidence for a structural lesion in the clinically suspected areas, only functional imaging revealed an obviously significant lesion. This case furthermore demonstrates that SPECT can contribute to the identification of the pathophysiology underlying visual hallucinosis.