AIM: The aim of this study was to evaluate in vivo the effectiveness of scaling and root planing of new oscillating instruments (Periosonic) using a sonic handpiece compared to hand curettes with a split mouth design after 2 months. METHODS: 11 patients with adult periodontitis participated in this study. Plaque index (PII) (O'Leary), bleeding on probing (BOP), probing pocket depth (PPD), recession (REC) and clinical attachment level (CAL) were recorded at baseline and 2 months after treatment. After oral hygiene instruction, 2 randomly assigned quadrants per patient were scaled and root planed with curettes (control side) and the remaining 2 quadrants with the Periosonic instruments 1 and 2 (test side). The student t-test for paired data was used to test the significance of difference between test and control sides. RESULTS: There was no statistical difference (p>0.05) between the 2 sides for the improvement of the clinical parameters excepted for the group with initial PPD of 4-6 mm (test: 1.3+/-0.4 mm PPD reduction, control: 1.6+/-0.4 mm). For PPD > or =7 mm, the test side had better clinical improvement in attachment levels (2.2+/-0.9 mm), less recession (-0.4+/-0.5 mm) with lower PPD reduction (2.4+/-0.6 mm) than the control side (AL: 1.6+/-1.8 mm; REC: -1.3+/-0.7 mm, PPD reduction: 3.0+/-1.4 mm). CONCLUSION: This clinical study demonstrated that Periosonic(R) instruments are clinically at least as effective as curettes in PPD reduction when initial PPD is < or =6 mm and show better clinical attachment level improvement with less recession for initial PPD of > or =7 mm.