A 10-year-old boy presented with generalized gingival inflammation, extensive alveolar bone loss, and tooth mobility. Clinical and radiographic examination supplemented by microbiologic investigation led to a diagnosis of classically termed prepubertal periodontitis (now known as generalized aggressive periodontitis). Other than severe periodontitis, the child was systemically healthy. Neither unusual infections nor abnormalities in neutrophil functions were detected. Microbiologic examinations by culture revealed the presence of the periodontal pathogen Actinobacillus actinomycetemcomitans. Treatment consisted of extraction of mobile teeth, supragingival and subgingival debridement, subgingival curettage, and root planing combined with a 1-week prescription of a combination of metronidazole and amoxicillin. Scanning electron microscopy of extracted teeth revealed hypoplastic and aplastic cementum at the periodontally exposed and intact surfaces. Clinical and microbiologic follow-up was continued over a 1-year period. No periodontal lesions have been detected, and A actinomycetemcomitans could not be isolated from the subgingival areas of the remaining teeth at the end of the first year. Since A actinomycetemcomitans was the main pathogen present in the subgingival microflora of the patient, it might play a key role in the etiology of prepubertal periodontitis.