The treatment of periodontally diseased root surfaces with power-driven instruments is becoming increasingly important as an alternative to the use of hand instruments. Root surfaces of artificial teeth on a manikin were debrided under simulated flap operation conditions to determine if effectiveness was dependent on the instrument used (curettes or a modified sonic scaler tip) and/or the operator's degree of experience. Two quadrants were treated with each instrument. A morphometric evaluation followed. Debridement was faster with the diamond-coated sonic scaler tip than with hand instruments. With the sonic scaler, the time necessary for tre atment was reduced by 30%. The experienced operators treated about 80% of the root surface, while the inexperienced operators treated only about 65%. Treatment qual ity depended on the operator and not on the instrument used.