It is not known whether changes in the thalamus play a role in the memory loss or dementia of Alzheimer's disease (AD), although trauma, infarction, and hemorrhage to the thalamus, particularly the dorsomedial nucleus (DMN), can cause these cognitive changes. To determine the pathologic changes in the DMN in AD, we examined the DMN in 16 cases of AD and 7 age-matched controls, with quantitative assessments of the total neuronal population and synaptic density, Alz-50-positive neurons, neurofibrillary tangles (NFT), and senile plaques (SP). We examined sections after staining with cresyl violet, a silver stain, and immunocytochemical staining for Alz-50 and synapsin I. Stereologic analysis demonstrated a mean loss of 29% of DMN neurons in AD and a synaptic density decrease of 21%. Alz-50 staining and NFT were present in all AD cases but in none of the controls. Senile plaques were 52 times more frequent in the DMN in AD than in the age-matched controls. The large variation in pathologic changes among our AD cases suggests that neuronal losses and other pathology in the DMN in AD may contribute to the total brain burden of pathology resulting in dementia in some AD patients, but not in others.