Maternal use of folic acid in early pregnancy has lowered the risk of some birth defects, notably neural tube defects. It is not clear whether it also lowers the risk of imperforate anus, a defect resulting from incomplete hindgut formation. The anus is either absent or abnormally located at about 8 weeks' development, with the rectum either ending in a blind pouch or opening into the urethra, bladder, or vagina. In the course of a public health campaign carried out in China from 1993 through 1995, pregnancy outcomes at 20 weeks' and more gestation were assessed in women asked to take one 400-pg pill, without other vitamins, each day from the preconception exam until the end of the first trimester. The study group of singleton pregnancies, totalling 222,314, came from one northern province and two southern provinces of China. Fifty women had a fetus or infant with imperforate anus; males predominated in a ratio of 1.66 to 1. Multiple anomalies were present in 16% of the group, and 22% had additional caudal anomalies such as persistent cloaca. None of the mothers had a history of diabetes. Imperforate anus was less frequent in offspring of women taking folic acid, with a risk ratio of 0.5 (95% CI, 0.29-0.88). The risk could not be related to geographic area, education, or occupational category. Higher parity and maternal age were associated with a greater risk of imperforate anus; controlling for either factor had only a small effect on the relative risk estimate. When adjusting for maternal age, the risk reduction among pill-takers was 41% (relative risk, 0.59). The United States and other nations have recommended that all women who might become pregnant take 400 μg of folic acid daily to lower the risk of neural tube defects. This measure may also help to reduce the risk of other defects such as imperforate anus.