The role of physical factors in the severity of labour pain has been neglected. The amount of cervical dilation, the frequency of the contractions, the woman's height and usual weight before the pregnancy and other physical factors were therefore examined in relation to the intensity of labour pain in 141 primiparous and 99 multiparous women. In general, pain increased gradually during labour in both groups of women, though the severity of the pain was lower in the women who had received prepared childbirth training than in those who had not. Although the average pain scores in this study were high, there were striking individual differences, some women having extremely severe pain and others having almost none. The pain scores in both groups of women were significantly correlated with the ratios of the women's usual weight to height. In the multiparous women the scores were also correlated with the woman's usual weight and the baby's weight but not with the woman's weight gain during pregnancy. Thus, the results show that physical as well as psychologic factors contribute to the severity of labour pain.