Host immune responses and the predominant subgingival microflora were evaluated in a 13-year-old female exhibiting a severe form of generalized juvenile periodontitis. The patient's neutrophils were chemotactically depressed but exhibited a normal oxidative capacity. Serum IgG antibody to Bacteroides gingivalis and to Actinobacillus actinomycetemcomitans serotype c were elevated. Significantly, B. gingivalis constituted 8 to 16% of the cultivable microflora and 13 to 20% of the total cell count in subgingival plaque samples obtained from five out of five periodontally diseased sites examined. It was not detectable in a healthy site. A. actinomycetemcomitans was recovered in small numbers from all subgingival plaque samples taken. The present study provides additional evidence for an etiologic association between B. gingivalis and generalized juvenile periodontitis.