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It has recently been suggested that systemic pethidine is ineffective in relieving labour pain. This study aims to evaluate the analgesic efficacy of pethidine in labour.This is a double blinded randomised placebo-controlled study.Labour suite, Prince of Wales Hospital, Hong Kong.Fifty normal Chinese pregnant women in early labour.We planned to randomise 112 women in early labour to receive either intramuscular pethidine 100 mg or normal saline when they requested analgesia. An interim analysis was performed at sample size 50.The primary outcome assessed was the visual analogue scale (VAS) pain score after 30 minutes.The study was terminated after recruitment of the first 50 women when planned interim analysis showed a significantly greater reduction of VAS pain score in the pethidine group (P= 0.009). The median difference in VAS pain score between the two groups at 30 minutes was −17 [95% CI −30 to −4]. Mean time to first subsequent request for analgesia was greater in the pethidine group (232 minutes, 95% CI 135 to 329 minutes) compared with the control group (75 minutes, 95% CI 54 to 95 minutes). Eight women (32%) in the pethidine group needed no further analgesia vs one woman (4%) in the control group (P= 0.011). Women in the pethidine group gave greater scores for sedation and satisfaction. Neonatal outcome was similar.Systemic pethidine was more effective at relieving labour pain than placebo. Its analgesic effect, however, was modest.