In a pilot study of a randomized controlled trial comparing pushing techniques in the second stage of labour, a surprise finding was that there was a positive correlation between the amount of pethidine used for analgesia in the first stage of labour and an increasing length of both the first (r = 0.5687, P = 0.0001, CI = 0.33 to 0.74) and second stages (r = 0.3204, P = 0.037, CI = 0.03 to 0.56). In order to investigate this further a review of the literature on the effect of pethidine on the length of labour was undertaken. The literature searched was the English-language literature, and MEDLINE and Index Medicus were used to identify pertinent papers. Studies selected were randomized controlled trials of pethidine given for pain relief in labour compared with placebo. As only five studies were identified other pertinent studies using the drug were scrutinized. The findings of this review are that, due to methodological flaws and studies with small sample sizes, the effect of pethidine on the length of labour in women has not been adequately assessed. However, there is a strong suggestion in the literature that the use of this drug is associated with a lengthening of labour and this association is dose-related. Studies in animals support this view. Those caring for women in labour should be aware of this side-effect of the analgesic most frequently given in labour in North America and the United Kingdom. As pethidine frequently does not provide adequate analgesia and has other side-effects, the search for an alternative analgesia should continue.