Introduction: The aim was to assess the association between alcohol intake during pregnancy and preterm delivery. Materials and methods: Women attending routine antenatal care at Aarhus University Hospital, Denmark, from 1989-1991 and 1992-1996 were eligible for the study. Our analyses included 18,228 singleton pregnancies. We obtained prospective information on alcohol intake at about 16 and 30 weeks of gestation, other lifestyle factors, maternal characteristics, and obstetrical risk factors from questionnaires completed by the women and hospital files. Results: For women with an alcohol intake of 1-2, 3-4, 5-9, and ≥ 10 drinks/week, the risk ratio (RR) of preterm delivery was 0.91 (95% CI: 0.76-1.08), 0.86 (0.64-1.15), 0.89 (0.52-1.52), and 2.93 (1.52-5.63) respectively, compared with an intake of <1 drink/week at about 16 weeks of gestation, and 0.69 (0.56-0.86), 0.82 (0.60-1.13), 0.97 (0.58-1.64), and 3.56 (1.78-7.13) at about 30 weeks. Adjustment for smoking habits, caffeine intake, age, height, prepregnant weight, marital status, occupational status, education, parity, chronic diseases, previous preterm delivery, mode of initiation of labour, and sex of the child did not alter the conclusions, nor did restriction of the highest intake group to women drinking 10-14 drinks/week (RR=3.41 (1.71-6.81) at 16 weeks and RR=3.47 (1.64-7.35) at 30 weeks). Discussion: The association between alcohol intake and preterm delivery appeared to be J-shaped, with a threshold for adverse effect at a level of about 10-14 drinks/week.