10 adult patients with periodontitis were treated with oral hygiene instruction and a single episode of supra- and subgingival debridement using either a sonic or an ultrasonic instrument in a split-mouth design. The clinical response was evaluated by measurements of dental plaque, bleeding on probing, probing depths, and probing attachment levels taken at baseline and every 3rd month for 12 months. An improvement of periodontal conditions was observed during the initial 3-6 month period followed by a stabilization of parameters. No difference in clinical response could be observed between sites treated with the sonic or ultrasonic instruments.