The object of the study was to determine the relation between cervical conization and preterm birth. All Danish women with singleton pregnancies who gave birth to their first infant in 1982 and second infant in 1982-1987 were included in a register-based cohort study. Information on pregnancy outcome and cervical conization in 1977-1987 was obtained from the Medical Birth Register and the National Register of Hospital Discharges. It was found that in a cohort of 14.233 women, 170 had had cervical conization. Thirty-four had had cervical conization before the first delivery, 62 between the first and the second, and 74 after the second delivery. Women with cervical conization had a significantly higher risk of preterm birth. Also, women with subsequent cervical conization had a higher risk of preterm birth in previous pregnancies. However, the risk of preterm birth was higher in women with previous than with subsequent cervical conization. It is concluded that cervical conization is correlated with preterm birth. Since women with subsequent cervical conization are at increased risk of preterm birth in preceding pregnancies, other factors than the surgical intervention may contribute to the significantly increased risk of preterm birth.