Three groups of volunteers, totalling 125, lived for two years on strict diets so that comparisons might be made with regard to the sweeteners: fructose (F), sucrose (S), and xylitol (X). The sizes of the test groups were: S, 35; F, 38; X, 52. The average monthly amounts of the sugars consumed in a varied assortment of foods were: S, 2.2 kg; F, 2.1 kg; X, 1.5 kg. The highest daily doses of fructose and xylitol were 200 - 400 g (maximum 430 g xylitol). Serum samples were analyzed for several chemical parameters. The dietary regimens did not result in clinically significant changes between the sugar groups. The ability of X to produce osmotic diarrhoea and flatulence was found to depend on the individual physiological responses of each volunteer. In many cases no symptoms were found although high amounts (200 - 400 g) of X were consumed. All pregnancies and deliveries in the F and X groups were normal. Practically all the volunteers accepted the F and X foods (almost 100 varities) and adhered to the dietary regimen for two years. This was due in the main to the fact that most F and X products were comparable to those containing sucrose.