BACKGROUND In a prospective randomized double-blind study, the analgesic effect produced by sub-Tenon infiltration was compared with classic analgesic drugs in patients scheduled for posterior segment surgery under general anaesthesia. METHODS One hundred patients were randomized into two groups of 50. One group received sub-Tenon infiltration (group 1) with 3 mL of bupivacaine 0.50% by the surgeon before the end of the surgery and the other (group 2) received only classical analgesic drugs postoperatively. A visual analogue scale (VAS) (graded from 1 to 10) was used to assess pain. For both groups, when VAS was between 1 and 3 paracetamol (3 g/24 h) associated with ketoprofen (200 mg/24 h) was given; between 3 and 6 nalbuphine (0.2 mg/kg slowly intravenously repeated every 4 h if necessary) was given; and over 6 morphine was given. Morphine 1 mg was injected every 2 min until VAS below 3 was obtained. RESULTS All patients in group 2 (control) experienced pain in the recovery room period; however, no patient in group 1 required analgesic drugs in the first 6 h after the sub-Tenon infiltration. In the recovery room period, the VAS pain score in patients who received sub-Tenon infiltration (group 1) was 0.6 +/- 1.3 (mean +/- SD) compared to 3.4 +/- 2.2 in group 2. The difference was statistically significant (P = 0.000001). All patients in group 2 asked for analgesic drugs in the recovery room, some of whom required morphine. Despite the administration of drugs the pain score was statistically higher in group 2. Between the end of the recovery room period and the 6th hour, the VAS pain score in group 1 was statistically lower. From the 6th until the 24th hour, the pain score was not statistically significantly different between the groups. Regarding consumption of analgesic drugs from the recovery room until the 24th hour, the consumption of level 1 analgesic drugs (paracetamol, ketoprofen) and level 3 (morphine) was statistically lower in group 1 (P = 0.0009). The difference was not significant for level 2, probably because the number of patients was not sufficient. CONCLUSIONS Sub-Tenon infiltration with 3 mL of bupivacaine 0.50% offers excellent postoperative analgesia for about 6 h and is an excellent alternative to classical drugs. Furthermore, it is highly reliable and safe.