Proton NMR imaging of the brain is rapidly becoming established as a useful investigative tool in medicine. This paper examines the usefulness of the NMR parameters—spin-lattice relaxation time (T1) and proton density (PD)—in differentiating groups of patients with senile dementia of Alzheimer type (SDAT) and multi-infarct dementia (MID) from each other, and from elderly controls. T1 values increase with severity of dementia. NMR parameters may also be of use in localising regions of brain damage.