The purpose of this study was to evaluate the discriminability of apathy and depression by determining whether the relationship of these two dimensions of behavior varies in different diagnostic groups. Using the authors' Apathy Evaluation Scale and the Hamilton Rating Scale for Depression, we rated 123 subjects, mean age 72 years, who met research criteria for healthy elderly controls, left or right hemisphere stroke, probable Alzheimer's disease, and major depression. Elevated apathy scores unassociated with elevated depression were most frequent in Alzheimer's disease and right hemisphere stroke, and also occurred in a small number of left hemisphere stroke and normal subjects. In major depression, apathy was associated with high depression scores, although a substantial number of major depressives showed elevated depression without elevated apathy. In left hemisphere stroke, probable Alzheimer's disease, and major depression, there were significant positive correlations between apathy and depression. The slope of the regression of apathy on depression was greatest in probable Alzheimer's disease and major depression. These results indicate that the relationship between apathy and depression differs across diagnostic groups and, thus, support the discriminability of apathy and depression.