A study utilizing a split-mouth design in 18 subjects and with four operators was performed in order to compare the effect of hand and ultrasonic instrumentation of periodontal pockets. Contralateral pairs of teeth with pockets of comparable depth were treated by either hand or ultrasonic instruments. Instrumentation was repeated after 4 weeks. Pocket depth, bleeding on probing and gingival fluid measurements were used for evaluation. A gradual reduction of pocket depth and number of bleeding points took place throughout the 8-week experimental period. Clinically significant differences were not observed comparing the effect of hand instruments to ultrasonics for any of the operators. The amounts of gingival fluid were similar for both methods of instrumentation at the final examination. Thus, the present study failed to demonstrate any significant differences between the effect of ultrasonic root debridement and hand instrumentation in the treatment of 4-6 mm deep periodontal pockets.