We investigated the relation between cobalamin deficiency, clinical changes and brain function in dementia patients. On admittance to the clinic, 24 patients had cobalamin deficiency, and dementia with additional symptoms of delirium. During cobalamin supplementation, the patients underwent repeated regional cerebral blood flow (rCBF) studies, psychiatric evaluations, and in some cases assessment with MMSE and the Organic Brain Syndrome scale. Fifteen patients who showed mild to moderate dementia improved clinically, and also showed a concomitant increase in their general CBF after treatment. In contrast, 9 patients who were severely demented showed no obvious clinical improvement, and no general blood flow change, although some regional flow increases were seen in sensory motor areas. We conclude that symptoms which probably indicated superimposed delirium such as clouding of consciousness, disorientation and clinical fluctuation, responded to the vitamin B12 supplementation, while the underlying dementia condition remained basically unchanged. The clinical improvement was also mirrored in general and focal rCBF changes.