It has been shown that patientswith localized juvenile Periodontitis (LJP) often harborActinobacillus actinomycetemcomitansin the subgingival area. However, little is known of the oral microflora in non‐LJP juvenile Periodontitis patients with less extensive disease. The purpose of this study was to describe the microflora and clinical parameters of young adults with minor to moderate periodontitis during treatment for a period of 1 year. Eleven patients 15 to 16 years of age were studied. All of them had 4 to 8 mm loss of attachment at minimally one site, but the typical clinical description of localized juvenile periodontitis was an exclusion criterion in this study. Microbiological examination of the deepest periodontal pocket and of the tongue revealed that 6 patients harboredActinobacillus actinomycetemcomitansand 5 harboredPorphyromonas gingivalis. Almost all subjects showed relatively high proportions ofPrevotella intermedia, Campylobacter rectus, motile organisms, and spirochetes. On the basis of clinical and microbiological parameters the 11 patients could be assigned to 1 of 2 groups. Six cases had moderate periodontal breakdown with loss of attachment at 7 to 44 sites. All harboredA. actinomycetemcomitansand 5 of themP. gingivalis. These 6 cases responded relatively well to initial treatment despite the continued presence of A. actinomycetemcomitans. The other group consisted of 5 cases with relatively minor periodontal breakdown; i.e, 1 or 2 sites with 4 to 6 mm loss of attachment. NeitherA. actinomycetemcomitansnorP. gingivaliswas detected in the deepest pocket of these patients. All 5 responded well to initial treatment. It can be hypothesized that the subgingival presence ofA. actinomycetemcomitansin non‐LJP patients is not predictive for a poor response to initial treatment. Therefore, recognition and treatment ofA. actinomycetemcomitansassociated periodontitis, if diagnosed in an early stage, may be possible to treat with conventional mechanical periodontal treatment. It remains to be determined if the presence of A. actinomycetemcomitans in these patients is a risk factor for further breakdown.J Periodontol 1993;64:40–47.