This study examines the effect of a chlorhexidine irrigant during ultrasonic scaling and root planning and compares clinical results with those obtained using a water irrigant. The irrigant was delivered either through a conventional or a recently-introduced ultrasonic scaler, which allows the irrigant solution to be passed through the inside of the scaler tip. The study utilizes a split-mouth design and contrasts the effectiveness of both instruments in improving periodontal health, as measured by standard clinical indices. A preliminary investigation of patient tolerance to treatment performed throughout the clinical trial was also undertaken. Statistical analysis at the 3 and 6 month post-treatment stage revealed no significant differences between the 4 experimental groups in respect of probing attachment levels (P > 0.5), bleeding index (P > 0.1), or plaque index (P > 0.05). Clinical attachment gain assessed by probing ranged from 0.6 to 0.9 mm. Chlorhexidine increased patient tolerance in 77% of cases and the new model ultrasonic scaler was favored over the traditional model in 70% of cases. The magnitude of the tolerance differences was however not statistically significant. The possible potential of the new scaler and of chlorhexidine as an irrigant to reduce patient discomfort is worthy of further investigation than was possible in this study.