The purpose of the present research was to examine the effect of non-surgical treatment on clinical and microbiological parameters on 126 periodontitis patients of the Depart-ment for clinical periodontology on Bernhard Gottlieb University Clinic of Dentistry Vienna. Measurements for clinical (pocket depth, bleeding on probing, plaque accumu-lation) and microbiological parameters (checkerboard DNA-DNA hybridization) were made before and after conventional periodontal therapy. Radiological provable bone loss was measured by the, for this diploma thesis developed Image J algorithm, based on the OPG pictures. The differences between clinical and microbiological parameters, before and after the therapy, were looked for with the help of the Wilcoxon test for paired samples. The connection between number of bacteria and average radiological provable bone loss were examined on 261 periodontal patients with the help of the rank coefficient of correlation according to Spearman. Statistic analysis (Wilcoxon test) for clinical and microbiological parameters showed a statistically significant difference between the results of the two measurements. That fact shows that the non-surgical pe-riodontal therapy was effective in the reduction of the bacterial counts and in the reduc-tion of the clinical parameters. The results of the rank coefficient of correlation accord-ing to Spearman showed that the correlations between analyzed characteristics are nega-tive. Therefore one cannot prove a statistically significant correlation between the bac-terial number and radiological provable bone loss.