PIP: The frequency of drinking alcohol among 616 women who aborted spontaneously (cases) was compared with that among 632 women who delivered after at least 28 weeks gestation (controls). 17.0% of cases reported drinking twice a week or more during pregnancy whereas among controls, only 8.1% of women reported drinking twice a week or more. The hypothesis that drinking during pregnancy is associated with spontaneous abortion was tested by maximum-likelihood logistic regression analysis. The adjusted-odds ratio for this association was 2.62. We estimate that more than 1/4 of pregnant women drinking twice a week or more are likely to abort, compared with about 14% among women who drink less often. Consideration of wine, beer, and spirits suggested that the minimum harmful dosage was 1 ounce of absolute alcohol. Several potentially confounding variables, including maternal age, gestation, prior spontaneous abortions, smoking, and nausea/vomiting, were controlled in the analysis. The association between drinking during pregnancy and spontaneous abortion did not vary with these factors. Even moderate alcohol consumption during pregnancy is a risk factor for, and may be a cause of, spontaneous abortion. Among the possible mechanisms, acute fetal poisoning seems the most likely, although chronic poisoning is also possible.