Our previous studies have shown that incomplete white matter infarctions are common in senile dementia of the Alzheimer type. To visualize and interpret these changes on magnetic resonance imaging (MRI), knowledge of MR relaxation times associated with this histopathology is important. The proton MR relaxation times T1 and T2 were determined in vitro for 3 groups of specimens. Fifty specimens of normal and incompletely infarcted white matter from 21 patients with senile dementia and 38 normal and pathologic white matter specimens from 19 elderly cases with infarcts were analyzed. The specimens were evaluated for infarct type, grade of incomplete infarction, and the proportion of old complete or incomplete infarction versus normal tissue in each sample. The incomplete infarct had longer relaxation times than normal white matter, with prolongation of T1 and T2 being proportional to the severity of the tissue changes. Old complete infarcts had longer relaxation times than both normal white matter and incomplete infarctions. These differences in relaxation times, reflecting various degrees of tissue damage, are important for the interpretation of MR images in vivo. Visualization of incomplete infarctions is essential for an in vivo diagnosis of these frequent and sometimes extensive changes.