The aims of the present study were (1) to analyze the separate effects on plaque, caries, and gingivitis of several measures (oral hygiene instruction, professional tooth cleaning with mechanical instruments, topical application of sodium monofluorophosphate - MFP) which have been incorporated in a prophylactic treatment program, described by AXELSSON & LINDHE1, and (2) to prevent caries and gingivitis by applying the combined treatment but replacing the professional mechanical tooth cleaning with chemical cleaning using a chlorhexidine-containing gel. 164 children, 13-14 years old, divided into four groups, participated in a 2-year trial. All participants were recalled once every 2nd week for preventive treatment provided by dental nurses. During the 1st year Group 3 and 4 received prophylactic treatment identical to that described by AXELSSON & LINDHE1. During the 2nd year the professional tooth cleaning was omitted but oral hygiene instruction was given every 2nd week. During the 1st year Groups 1 and 2 were treated with 0.5% chlorhexidine but during the 2nd year they were subjected to professional tooth cleaning with mechanical instruments. Throughout the trial Groups 1 and 3 rinsed once every 2nd week with a 2% MFP solution. The results showed that by oral hygiene instruction and frequently repeated professional tooth cleanings it was possible to substantially reduce the frequency of gingivitis and prevent caries. When this treatment was used, mouthrinsings with 2% MFP had no additional effect on caries. The substitution of the mechanical cleaning procedure with topical application of a 0.5% chlorhexidine gel failed to (1) remove supragingival plaque accumulations, (2) reduce the frequency of gingivitis, and (3) retard the rate of caries development. The data also revealed that interdental cleaning with mechanical instruments was of decisive importance in the prevention of approximal surface caries.