The purpose of the paper is to describe the essential findings in two clinical trials, carried out in order to evaluate differences in the caries increment rate as influenced by various sugars. The first study involved almost complete substitution of sucrose (S) by fructose (F) or xylitol (X) during a period of 2 years. The second study comprised partial substitution, the effects of a S- or X-containing chewing gum being compared during 1 year. In the first study there were no significant differences initially between the sugar groups; 35 subjects in the S-group, 38 in the F-group, and 52 in the X-group. During the entire study 10 subjects discontinued or were excluded. After 2 years the mean increment of decayed, missed and filled tooth surfaces was 7-2 in the S-group, 3-8 in the F-group, and 0-0 in the X-group. The second study comprised initially 102 subjects, predominantly dental and medical sutdents, divided randomly into S- and X-groups. During the study 2 subjects were excluded. The subjects consumed 4-0 chewing gums per day in the S-group and correspondingly 4-8 in the X-group. The frequency of sucrose intake was 4-24 times per day in the S-group, and 4-94 in the X-group. The caries incidence assess independently by clinical and radiographical means, equalled the 1-year observations in the first study. The results show a massive reduction of the caries increment not only in relation to total, but also in connection with partial substitution of dietary sucrose and low xylitol consumption. The first study indicates fructose to be somewhat less cariogenic than sucrose. In view of the findings in the chewing gum study it is suggested that the non- and anticariogenic properties of xylitol principally depend on its lack of suitability for microbial metabolism and the physico-chemical effects in plaque and saliva brought about through low and repeated dosage.