Objectives: Certain human leucozyte antigens (HLA) could influence thesubgingival colonization with periodontopathic bacteria. The aim of thepresent study was to evaluate HLA associations to A. actinomycetemcomitans(A.a.), P. gingivalis (P.g.), P. intermedia (P.i.), T. forsythensis (T.f.)and T. denticola (T.d.) in patients with aggressive periodontitis (AP) (N =33, age 37 years) and patients with chronic periodontitis (CP) (N = 35, age47 years). Methods: In both patient groups the smoking status, theapproximal plaque index (API), bleeding on probing (BOP) and the clinicalattachment loss (CAL) were determined. Bacterial infection was analyzedemploying a PCR-rSSO microDent DNA-Strip technique (Hain Lifescience GmbH,Nehren, Germany). HLA-A, -B, -Cw, -DR, -DQ typing was performed by bothComplement Depending Cytotoxicity assay, BAG, Lich, Germany) and PCR-SSP(GenoVision VertriebsmbH Schwechat, Austria). Statistical calculations werecarried out by Chi²-testing, with Yates correction or Fishers Exact test, ifappropriate. Results: The occurence of an A.a. infection was found to bepositively associated with the haplotypesHLA-B*08:Cw*07:DRB1*03:DRB3*(DR52):DQB1*02 as well asHLA-DRB1*13:DRB3*:DQB1*06, whereas a negative association was found toHLA-DRB1*04:DRB4*:DQB1*03. HLA-A*02 positve patients were always infectedwith P.g.. An infection with P.i. occurred less frequent in patientsexpressing HLA-B*5, HLA-DRB1*13, -B*44:DRB1*13 or-DRB1*13:DRB3*(DR52):DQB1*06. T.f. was negatively associated with HLA-A*33. Conclusions: Single HLA markers and HLA combinations appear to influence thesusceptibility for an infection with periodontopathic bacteria. In thiscontext, it may be of interest that both HLA-B*08 andHLA-DRB1*04:DRB4*(DR53) positive haplotypes are also associated with severalother chronic inflammatory or autoimmune diseases. Keywords: HLA, aggressive periodontitis, chronic periodontitis, bacteria, subgingivalcolonization