There are differing opinions as to the extent to which root cementum has to be removed during root surface instrumentation over and above that of the debridement of plaque and calculus. Similarly, the amount of tooth material removed by individual instruments is also unclear, but a trend towards less damaging methods of root surface debridement has evolved in recent years. The purpose of this in vitro study was to determine the amounts of root substance removed by 4 different methods of instrumentation, hand curette, ultrasonic scaler, airscaler and fine grit diamond bur. Measurement of tooth substance loss was carried out with a specially constructed measuring device at 360 sites on 90 mandibular incisors following 12 working strokes with a clinically appropriate force of application. Only a thin layer of root substance (11.6 microns) was removed by the ultrasonic scaler, compared to the much greater losses sustained with the airscaler (93.5 microns), the curette (108.9 microns) and the diamond bur (118.7 microns).