The effect of thiamin supplementation on the health and general well-being of 80 randomly selected healthy elderly Irish women, from a population with marginal thiamin deficiency, was studied. Key variables affecting thiamin status were controlled. Weekly dietary intakes, subjective feelings, and activity assessments were measured during a 4-week baseline and 6-week double blind treatment period. Clinical assessments were performed during the last week of each period. For treatment, subjects were randomly assigned to either thiamin (10 mg daily) or placebo groups. Compared to baseline and placebo supplemented values, thiamin-supplemented women experienced significantly increased appetite, energy intake, body weight and general well-being, and decreased fatigue. Thiamin supplementation also tended to reduce daytime sleep time, improve sleep patterns, and increase activity. These data suggest that evaluation of thiamin status is indicated when nonspecific conditions such as anorexia, weight loss, fatigue, depression, and sleep disorders are present in elderly persons.