The purpose of the investigation was to estimate the relative effect of fluoride on caries in a preventive program based on meticulous plaque control. The material consisted of 82 children; 41 in the test and 41 in the control group. At the start of the study the children were 13-14 years of age. In August 1973 an examination was performed to obtain baseline data for the trial. Prophylactic treatment was given to the children once every second week. A total of 18 treatments were given during a 10-month period. In the control group an abrasive paste was used containing 5% sodium monofluorophosphate (Jodka Fluor Polerpasta). In the test group an abrasive paste was used which was identical with the control paste except that it did not contain any fluoride components. Twelve months after the baseline examination the test and control groups were reexamined. The trial demonstrated that, in children, fortnightly professional cleaning of the teeth combined with meticulous toothbrushing instructions result in (1) a high standard of oral hygiene and (2) only very few new carious surfaces. Though the control group children received fortnightly topical application of F- during the prophylactic sessions and used a fluoridated dentifrice, there was no significant difference between the two groups regarding number of new carious tooth surfaces per child per year.