A comprehensive new oral hygiene training program has lately been described and found effective in controlling dental plaque formation and in significantly reducing the incidence of approximal dental caries and gingivitis in adolescents. This study investigated the long-term effect of plaque control on the progression of periodontal diseases in adolescents. A group of 227 Brazilian schoolchildren were followed up over a period of 3 years. The children were divided randomly into 3 groups. The first group was given a needs-related intensive program which combined detailed information to parents and children pertaining to the etiology and prevention of dental diseases, instructions in self-diagnosis of plaque and gingivitis, and a detailed oral hygiene training based on individual needs, together with continued feedback and motivation during the entire 3 years. The second group was given a similar program, but through shorter sessions and with no motivation and feedback and no training in self-diagnosis. A control group received no motivation sessions or oral hygiene training. The 3 groups were examined radiographically at baseline and annually at 3 subsequent occasions. The data were analyzed with a multi-level variance analysis. The frequency of subjects showing sites with alveolar bone loss increased steadily during the entire period in all groups. Neither of the training programs had a significant effect on the alveolar bone level during 3 years compared to the control group. Generally, girls demonstrated higher proportions of approximal tooth surfaces showing radiographic bone loss, though not statistically significant. In this population the supragingival plaque control lacked any significant effect on periodontal disease progression.(ABSTRACT TRUNCATED AT 250 WORDS)