A detailed study has been made of uterine activity in a group of 2 1 unselected women in spontaneous labour. Measurements were made of contraction frequency, active pressure, Montevideo units and active contraction area. The level of Montevideo units were comparable with values previously reported in spontaneous labour. Active contraction area correlated better than any of the other measures with the rate of cervical dilatation in the active phase of labour (r = 0.69). The mean contraction area was 1099 (SD 333) kPas/lS min.