In 1971-72, a total of 375 adult subjects were recruited for a clinical trial aimed at assessing the effect of a preventive program, based on plaque control and topical application of fluoride, on the incidence of caries and periodontal disease. After a baseline examination, the volunteers were subjected to scaling, root planning and conventional caries therapy. During the course of the subsequent 6 years, they were recalled for preventive measures once every 2-3 months. After the 6-year follow-up examination, however, it was decided to extend the interval between the preventive sessions. Thus, during the next 9-year period, about 95% of the participants returned for preventive measures only 1 to 2 times per year. A small subgroup of about 15 subjects, who, during the initial 6 years had developed new caries lesions or had exhibited additional periodontal attachment loss, however, were also during the following 9 years recalled 3-6 times per year for oral hygiene control and preventive therapy. The re-examination performed in 1987 disclosed that the 317 subjects, who participated during the entire 15-year period, had a low incidence of caries and almost no further loss of periodontal tissue support. It was suggested that improved self performed oral hygiene, daily use of fluoridated dentifrice and regularly repeated professional tooth cleaning effectively prevented recurrence of dental disease.