Women attending routine antenatal care at Aarhus University Hospital, Denmark, from 1989 to 1996 were eligible for this cohort study on the association between alcohol intake in pregnancy and the risk of spontaneous abortion. We included 24 679 singleton pregnancies in the study. We used Cox regression analyses with delayed entry for the analyses. We found an increased risk of spontaneous abortion in the first trimester (7-11 completed weeks of gestation) for women consuming > or =5 drinks/week. We found no association between alcohol intake and spontaneous abortion during the second trimester. Common methodological problems in relation to studies on spontaneous abortions are discussed, as are problems with possible under-reporting of alcohol consumption. We conclude that women consuming > or =5 drinks/week are at increased risk of first trimester spontaneous abortion.