BACKGROUND: Periodontal bacteria are known to invade the systemic circulation. Chronic low-level bacteremia and a systemic inflammatory response have been suggested as a pathogenetic link between periodontal disease and atherosclerosis. The purpose of this study was to examine the systemic effect of a non-surgical therapy on white blood cell count (WBC count) and differential blood count in smoking and non-smoking generalized aggressive periodontitis (GAP) patients. METHODS: 27 adult periodontitis patients (13 smokers and 14 non-smokers) with previously untreated GAP were subjected to 3 sessions of oral hygiene procedure. Afterwards, the patients were treated by scaling and root planing under local anaesthesia. Periodontal examinations were performed after supragingival pretreatment and three months after subgingival therapy. Pocket probing depth (PPD) and relative attachment level (RAL) were measured with Florida probe and disc probe. Accompanying clinical evaluation venous blood samples were taken to analyse the WBC counts and differential blood counts. For statistical analysis non-parametric tests were utilized. RESULTS: No clinical or demographic differences were found between smokers (n=13) and non-smokers (n=14). PPD, bleeding on probing (BoP) and suppuration improved significantly after therapy both in smokers and non-smokers. Following periodontal treatment WBC counts, neutrophil and platelet counts decreased significantly in non-smokers (p<or =0.004), while in smokers only platelet counts were significantly reduced (p=0.006). Non-smokers showed a significantly higher reduction of WBC counts (p=0.005) and neutrophils (p=0.001) compared to smokers. CONCLUSION: The results indicate that a therapeutical intervention may have a systemic effect on the blood count in GAP patients. This effect seems to differ between smokers and non-smokers.