Results of various modalities of periodontal therapy were studied in 90 subjects (mean age 45 years) with moderate to severe periodontitis. Initial measurements of pocket depth and clinical attachment levels were compared with measurements obtained after the initial hygienic phase of the treatment and measurements of the same areas 1 and 2 years after four different types of periodontal treatment had been applied on a randomized basis to each of the four quadrants of the dentition. These treatments were: (1) surgical pocket elimination or reduction, (2) modified Widman flap surgery. (3) subgingival curettage, (4) scaling and root planing only. The patients were recalled for prophylaxis every 3 months, and rescored annually. One-way analysis of variance and Scheffe's method were used to test the hypothesis of equal treatment effects. The results were analyzed both with initial pocket depth as the baseline and with pocket depth at the hygienic phase as the baseline using a grouping of pockets 1 to 3 mm, 4 to 6 mm, and greater than or equal to 7 mm. For the 1 to 3 mm pockets there was a slight reduction in depth at the hygienic phase, with only minor changes after the various modalities of treatment over 2 years. However, significant losses of attachment after all modalities of periodontal therapy, including scaling alone, were observed at both the 1-year an 2-year intervals. For pockets 4 to 6 mm deep, the main reduction in pocket depth occurred at the hygienic phase, but the pockets also were reduced by further treatment, most by pocket elimination and modified Widman surgery. However, this reduction in pocket depth after surgery had no beneficial influence on maintenance of the attachment level, which actually was maintained best by scaling alone. For deep pockets greater than or equal to 7 mm, significant reduction in pocket depth occurred both at the hygienic phase and 1 to 2 years after treatment, with the greatest initial reduction after pocket elimination surgery. However, again there was no significant difference in attachment results among the four methods.