In the treatment of chronic adult periodontitis, scaling and root planing have been generally performed prior to periodontal flap surgery. The purpose of this study was to evaluate the clinical significance of nonsurgical periodontal therapy prior to periodontal flap surgery in patients with chronic adult periodontitis. Fifty six molars showing bilateral bony defects and 4-6mm periodontal pocket in twelve patients with chronic adult periodontitis were selected. By randomized split-mouth design, in one side, flap operation was performed 4 weeks after scaling and root planing, in the other group, flap operation was only performed without scaling and root planing. Probing pocket depth, gingival recession, clinical attachment level, gingival index, plaque index, and tooth mobility were measured at baseline, before flap operation and post-operation