In our previous study, we demonstrated that supervised daily mouth-rinsing program with a weak fluoride solution (F: 100ppm) based on a six-year school dental health service (in an elementary school) had a significant cariostatic effect. When the participants in this program left school, however, they had to discontinue the fluoride mouth-rinsing. It is, therefore, necessary to assess the durability of the cariostatic effect obtained through this program. As the subjects for present study, the children who took part in this program (the fluoride group) and others who were given the same school dental care without the fluoride mouth-rinsing (the control group) were chosen. Three years after they left their elementary schools, they were given a dental examination. During this three-year period, no organized prophylaxis had been given to them. The routine dental examination was the same as that used by us in previous reports.The results may be summarized as follows:1. The new DMFT increment during the three years after the discontinuing of the fluoride mouth-rinsing was 1.76 in the fluoride group and 2.58 in the control group, showing a caries reduction of 31.8%.2. The new DMFS increments on different tooth surfaces during this three years were analyzed, showing a caries reduction of 71.9% on smooth surfaces and 20.6% on other surfaces with pits and fissures.3. No cariostatic effect was found on the second molars, especially in the newly erupted teeth.4. This means that there was no cariostatic effect on teeth with little exposure to fluoride.It is evident from this investigation that there was a prolonged cariostatic effect during the three years after discontinuing of the weak fluoride mouth-rinsing. Therefore, we suggested that the fluoride mouth-rinsing program should be continued longer, to protect the second molars from dental caries.