The development of periodontal disease in Down syndrome adolescents (n = 34) was studied clinically and on intraoral radiographs during a 7-yr period. The occurrence of gingival inflammation (GBI), pathological periodontal pockets (>4 mm), sub- and supragingival calculus, alveolar bone height, alveolar bone loss, and the occurrence of the periodontal pathogens Actinobacillus actinomycetemcomitans, Capnocytophaga, and Porphyromonas gingivalis in subgingival plaque were determined. Of the subjects, 41% had one or more pathological periodontal pockets at baseline compared to 65% at follow-up. At the baseline examination, 35% of the individuals exhibited alveolar bone loss compared to 74% at the follow-up. The median value of sites with alveolar bone loss increased from 0 to 1, the new lesions mainly being located in the incisor region. The estimated annual reduction of alveolar bone height in each subject was 0.04 mm on average. The occurrence of the periodontal pathogens A. actinomycetemcomitans, Capnocytophaga, and P. gingivalis in subgingival plaque did not differ between baseline and follow-up. The results of the present study indicate that the frequency of periodontitis, mainly located on the lower incisors, markedly increased during a 7-yr period in Down syndrome individuals, although the severity and progression was limited compared to what has previously been described.