We evaluated the association between alcohol intake during pregnancy and preterm delivery. Women attending routine antenatal care at Aarhus University Hospital, Denmark, from 1989-1991 and 1992-1996 were eligible. We included 18,228 singleton pregnancies in the analyses. We obtained prospective information on alcohol intake at 16 and 30 weeks of gestation, other lifestyle factors, maternal characteristics, and obstetrical risk factors from self-administered questionnaires and hospital files. For women with alcohol intake of 1-2, 3-4, 5-9, and > or =10 drinks/week the risk ratio (RR) of preterm delivery was 0.91 (95% CI = 0.76-1.08), 0.86 (95% CI = 0.64-1.15), 0.89 (95% CI = 0.52-1.52), and 2.93 (95% CI = 1.52-5.63), respectively, compared with intake of <1 drink/week at 16 weeks gestation, and 0.69 (95% CI = 0.56-0.86), 0.82 (95% CI = 0.60-1.13), 0.97 (95% CI = 0.58-1.64), and 3.56 (95% CI = 1.78-7.13) at 30 weeks. Adjustment for smoking habits, caffeine intake, age, height, pre-pregnant weight, marital status, occupational status, education, parity, chronic diseases, previous preterm delivery, mode of initiation of labor, and sex of the child did not change 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).