The present study was performed to evaluate the relationship between the serotype or the genotype of Actnobacillus actinomycetemcomitans (A. a.) and the severity of periodontal disease. Total 64 A. a. clinical isolates were sampled from 46 sites of 20 subjects classified into the group I (1 periodontally healthy subject, 2 gingivitis patients, 5 ealry adult periodontitis patients), group II (3 moderatelly adult periodontitis patients) and group III (1 advanced adult periodontitis patient, 8 RPP patients). Southern bolt hybridization (fingerprinting) patterns of the five reference strains, A. a. strain ATCC 29523 (serotype a), ATCC 29522 (Serotype b), ATCC 43719 (serotype c), IDH 781 (serotype d) and IDH 1705 (serotype e), were used as the five basic genotypic patterns (A, B, C, D, E). NT type was designated as one which did dnot represent any of those five basic types. The serotypes were determined by ELISA technique with the serum samples from pre-immunized rabbit. Based on subject-based analysis, it was noted that genotypes A and C, NT, and B, D, E were significantly related to the disease groups I, II, and III, respectively. It was also noted that both the serotypes a and c were significantly related to the disease group I and II, while serotypes were significantly related bm), and serotypes b and nd were frequently found in sites with severe attachment loss (LA>6mm). The results indicated that the significant relationship can be delineated beteen the genotypes and the serotypes of Acinobacillus actinomycetemcomitans and the periodontal disease severity. The results also indicated that genotyping can provide more detailed information on its relationship with the disease severity based on both the patient-based and the site-based analyses.