Fifty-four elderly patients, living at home, completed a 3-month, double-blind, randomized, placebo-controlled trial of the effects of Tanakan on both cognitive efficiency and the quality of life. Patients were included in the study if they showed mild signs of impairment in everyday function on the Crichton Geriatric Rating Scale. Cognitive efficiency was measured before and at monthly intervals during the trial using a battery of tests of mental ability comprising both computerized and pencil-and-paper tasks. The quality of life was assessed using a behavioural questionnaire which was administered before and after the study. Individual analyses of the cognitive tests showed some advantage of Tanakan over placebo, but when the accuracy scores from all eight tests were combined using two different techniques, although both groups improved over time, the improvement with Tanakan at week 12 was significantly greater than with placebo. No signs of improvement were detected with placebo when the reaction time measures were combined using these techniques, whereas by week 4 the Tanakan group were significantly faster than at baseline, and were superior to placebo throughout the study. These improvements in mental efficiency were accompanied by a significant increase in the interest taken in everyday activities, and suggest that the drug might be helpful in treating the early stages of primary degenerative dementia.