Longitudinal studies have reported the effect of various modalities of periodontal surgery on pocket depth and attachment levels related to pretreatment measurements. However, possible changes in these measurements as a result of scaling, oral hygiene improvements and occlusal adjustment during the hygienic phase were not considered. The purpose of the present study was to examine the short-term effect of treatment of the hygienic phase in 90 patients with some pockets extending 4 mm or more apically to the CEJ. Pretreatment pocket depths and attachment levels related to the CEJ were measured by a thin probe in five sites at all 2,355 teeth in the sample. Scaling, root planing, instruction in oral hygiene and occlusal adjustment were completed during four to six sessions for each patient. Four weeks after completion of the hygienic phase, all variables were recorded. Mean measurements for pocket depths 1-3 mm, 4-6 mm, and greater than or equal to 7 mm prior to treatment were compared to their posttreatment scores. Pocket depth decreased significantly for pockets extending 4 mm or more apically to the FGM. For pockets 4-6 mm there was a mean difference in pocket depth of 0.96 +/- 0.47 mm (P < .0001) between pretreatment and posttreatment observations. For pockets 7 mm or greater the mean difference was 2.22 +/- 1.35 mm (P < .0001). Reduction in depth of pocket and improvement in attachment levels were related to the initial level of severity. Pocket reduction was in part due to the improvement in attachment levels. This study has demonstrated that the clinical severity of periodontitis is reduced significantly 1 month following the hygienic phase of periodontal therapy, and that need for surgical pocket treatment cannot be assessed properly until completion of the hygienic phase of treatment.