A clinical study was designed to evaluate the effect of frequent interdental flossing on the incidence of proximal dental caries. School children from a fluoride-deficient area were studied after clinical and radiographic examinations. Each child had at least one contralateral pair of intact, contacting proximal tooth surfaces between the distal surface of the primary cuspid and the mesial surface of the first permanent molar. Randomly selected test surfaces were flossed each school day with unwaxed dental floss by researchers. The contralateral surfaces served as controls. Flossing was done for eight months, discontinued for four months, and reinstituted for another eight months. A significant reduction in the incidence of proximal caries resulted.