A cohort of 22 948 women from a stable homogeneous population who gave birth for the first time between 1964 and 1983 were followed up prospectively. Analysis by mode of delivery showed that of those delivered by caesarean section 23·2% fewer had another pregnancy than those who had a spontaneous vaginal delivery. Women delivered by forceps were in an intermediate group. Miscarriage was more common in women who had been delivered by caesarean section. The relative infertility after caesarean section could not be accounted for by early sterilization, was not associated with maternal height or social status, and was only partly attributable to age.