Two studies were carried out in patients undergoing bilaterally symmetrical lower third molar surgery under general anaesthesia. In the first study observations were carried out in 40 patients, using McGill pain questionnaires and visual analogue scales (VAS), to assess the variation in pain intensity between the two sides in each individual on the morning after surgery. The within-patient variation in pain intensity indicated that a sample size exceeding 30 was required to show a difference of more than 10 mm on the VAS and achieve a power of 90% at the 5% level of significance. The second study was carried out to determine if local anaesthesia administered before rather than after tooth removal rendered any benefit. Each of the 38 patients acted as their own control; one side was allocated randomly to receive the local anaesthetic before operation and the other side after operation. The difference in pain between the pretreated and post-treated sides in each individual was assessed using a McGill pain questionnaire on the morning after surgery in addition to two VAS (one for each operation side) after operation, at 6 h, 1 and 6 days after surgery. There were no significant differences in pain at any time using any of the above methods.