The present study was performed to investigate the factors associated with disease progression in localized juvenile periodontitis (LJP) patients by longitudinal monitoring of microbiological changes. Following a 9-month period, 9 LJP patients were divided into 2 groups based upon attachment loss, progressing and non-progressing. Both groups received scaling, root planing, and modified Widman flaps. Clinical and microbiological data were obtained at baseline, following the observation period, and at 6 and 12 months post-treatment. At 6 and 12 months post-treatment significantly more cocci were persistent in the non-progressing group than in the progressing group. Actinobacillus actinomycetemcomitans was also more frequently isolated in the progressing group than in the non-progressing group initially and following the 9-month observation period. Also after treatment, A. actinomycetemcomitans recolonized earlier in the progressing group than in the non-progressing group. These studies suggest that A. actinomycetemcomitans may play a role in disease progression in LJP; however, they do not eliminate the possibility that other organisms may also play a role, since A. actinomycetemcomitans was not detected in all of the patients in whom disease progressed.