Sixteen patients with advanced periodontitis (more than 3 sites with a probing depth > or = 6 mm) were treated with the aim of eliminating Actinobacillus actinomyce-temcomitans and Porphyromonas gingivalis and reducing Prevotella intermedia below 5% of the total viable count. The patients were followed clinically and microbiologically for over 5 years and showed a highly individual pattern with respect to presence of indicator bacteria, type of treatment needed to accomplish treatment goals, clinical response, and bacterial and disease recurrencies. The results of this study suggest that presence of the indicator bacteria in microbial samples taken after treatment may identify patients at risk for recurrent periodontitis.