Healing events following nonsurgical periodontal therapy in patients with periodontal pockets up to 12 mm deep were investigated. Incisors, cuspids and premolars in 16 patients were treated by plaque control and supra- and subgingival debridement using hand or ultrasonic instruments in a split mouth approach. The results were evaluated by recording of plaque scores, bleeding on probing, probing pocket depths and probing attachment levels. Minimal change in gingival conditions occurred during the initial 3 months of experimentation, which were utilized for plaque control measures alone. Subsequent to instrumentation and during the following 9-month period, a gradual and marked improvement of periodontal conditions took place. During the remaining 12 months of the 24-month experimental period no further changes of the recorded parameters were noted. No differences in results could be observed when comparing hand versus ultrasonic instrumentation, or when comparing the results of 2 different operators. Initially, a total of 305 sites demonstrated probing pocket depths greater than or equal to 7 mm. At the 24-month examination 43 such sites remained. The results indicate that there is no certain magnitude of initial probing pocket depth where nonsurgical periodontal therapy is no longer effective.