The aim of the present study was to evaluate the effect of non-surgical periodontal therapy on probing pocket depths and probing attachment levels using a patient group with moderate to advanced periodontitis. 68 patients with moderate to advanced periodontitis underwent hygienic phase therapy including oral hygiene instructions, scaling and root planing and elimination of plaque retentive factors. Assessments of the plaque control record (PCR), bleeding on probing (BoP), probing depths and probing attachment levels were performed at baseline examination and 3 to 5 months following active treatment. The measurements were obtained at 4 interproximal aspects of each tooth with a thin calibrated probe. Mean BoP values decreased from 63.2 +/- 21.9% at baseline to 16.6 +/- 7.3% after therapy, and mean PCR decreased from 78.6 +/- 16.4% to 12.7 +/- 7.1%, respectively. A reduction in mean probing pocket depth from 3.96 +/- 1.39 mm at baseline to 3.30 +/- 1.16 mm after therapy was noted. Sites with initial probing depths of 1-3 mm showed no change, sites with initial values of 4-6 mm revealed a reduction of 1.03 +/- 1.04 mm, while initial pockets of 7-9 mm decreased in depth by 2.28 +/- 1.62 mm. A gain in the mean probing attachment level from 4.16 +/- 1.80 mm to 3.74 +/- 1.71 mm was observed as a result of treatment. The group with the shallow initial probing depths of 1-3 mm showed no alteration in probing attachment level. Pockets with baseline values of 4-6 mm showed gain of clinical attachment of 0.69 +/- 1.43 mm. The greatest gain in clinical attachment of 1.51 +/- 1.75 mm was obtained in sites with initially deep pockets of 7-9 mm. From the results of this study, it can be concluded that non-surgical periodontal therapy is an effective means to reduce probing pocket depths and to improved clinical attachment levels in patients with moderate to advanced periodontitis.