Objectives: To characterize A. actinomycetemcomitans isolates from young Chinese aggressive periodontitis patients. Methods: Subgingival plaque samples (2/subject) were collected from untreated diseased subjects < 25 yr (n = 9, 21.1 ± 1.6 yr) and age- and sex-matched periodontitis-free controls (n = 47, 22.0 ± 1.1 yr). Selective (TSBV agar) and anaerobic culture were used. The serotype, leukotoxin gene (ltx) operon promoter, and the cytolethal distending toxin (cdt) genes complex of the A. actinomycetemcomitans isolates were studied. Results: Diseased subjects had significantly higher full-mouth bleeding scores (p=0.002) and total viable counts (7.2x106 vs 2.1x105 c.f.u., p<0.005). A. actinomycetemcomitans was isolated from 67%/56% or 6%/4% of diseased or control subjects/sites, respectively (p<0.001). The proportion of A. actinomycetemcomitans isolatable from both diseased and periodontitis-free was low (0.7% vs 0.1%, p<0.02). Among the 10 disease and 4 control A. actinomycetemcomitans isolates characterized, 8 were serotype c, 3 serotype f, 2 serotype a and 1 serotype b, and all possessed 652-like ltx gene promoter; furthermore all but one from diseased patients had intact cdtABC genes. Conclusion: This study confirmed increased prevalence and quantity of A. actinomycetemcomitans in aggressive periodontitis Chinese patients. The ltx promoter and cdt characteristics of the A. actinomycetemcomitans isolates, however, were similar among the diseased and control groups.