The purpose of this study was to determine the effects of a dietary supplement (Fortisip, Cow & Gate Ltd) on the body weight, triceps skinfold thickness and mid-arm muscle circumference of low weight, elderly, demented hospital residents. The height and weight of all residents (n = 293) of the Care of the Elderly wards of a large psychiatric hospital were measured, and body mass index (BMI) calculated for each individual. Forty-six underweight residents (BMI 15.1-19.9) were matched by age, sex and BMI and randomly allocated to receive the supplement or an acaloric vitamin placebo. There were significant increases in mean weight (P < 0.001), mid-upper-arm muscle circumference (P < 0.05) and triceps skinfold thickness (P < 0.001) in the supplemented group (n = 23) None of these measures of nutritional status changed significantly in the placebo group (n = 23). A high proportion of elderly demented hospital residents are underweight. The addition of a dietary supplement produced a highly significant increase in mean body weight as a result of increases in both body fat and muscle protein.