The Set test is a simple rapid test of mental function. It was applied to a sample of 189 elderly subjects in the East End of Glasgow. A score of under 15 on the test corresponded closely to a clinical diagnosis of dementia. Scores in the range 15 to 24 showed a lesser degree of association with dementia; while no subject with a score of 25 or over was demented. Low scores in the test were associated with physical illness, and to a limited extent with low social class, but not with affective illness. The test appears worthy of further studies in screening programmes, and may have other clinical and epidemiological uses.