Replies to a questionnaire showed that, amongst 180 women delivered in The London Hospital, 48% experienced backache during pregnancy; in one third of these it was severe. The prevalence of back pain increased with both increasing age and increasing parity, and it was difficult to separate the relative contributions of these two factors. No evidence was found of an association between backache during pregnancy and height, weight, 'obesity index', weight gain, or baby's weight. Analysis of aggravating and relieving factors indicates some differences between backache in the pregnant and 'mechanical' back pain in the non-pregnant. Slightly less backache was reported amongst patients attending antenatal physiotherapy classes but the figures do not provide clear evidence of any protective effect of this attendance.