Low maternal serum alpha-fetoprotein values during the second trimester of pregnancy are associated with an increased risk of Down syndrome in the fetus. In this study a sensitive, monoclonal-based radioimmunoassay for alpha-fetoprotein was used to determine whether such an association also applies to the first trimester and if maternal serum alpha-fetoprotein screening could successfully detect a significant number of pregnancies in which the fetus had a trisomy or other chromosome disorder. Sera were obtained prospectively from 540 women just before chorionic villus sampling for prenatal diagnosis of chromosome defects (largely because of advanced maternal age) at 8 to 12 weeks' fetal age and assayed for alpha-fetoprotein under code without knowledge of the cytogenetic results. Eight of 27 (29.6%) of all serious chromosome defects were associated with low maternal serum alpha-fetoprotein values (less than or equal to 0.6 multiples of the median). Overall, 59 of 540 patients (10.9%) had maternal serum alpha-fetoprotein values less than or equal to 0.6 multiples of the median, eight of whom had a fetus with a serious chromosome defect. Women whose maternal serum alpha-fetoprotein value was less than or equal to 0.6 multiples of the median had one in eight odds of carrying a fetus with a trisomy and one in seven odds of the fetus having any serious chromosome defect. From this study of a group of women at higher risk, we conclude that first-trimester maternal serum alpha-fetoprotein screening for chromosome defects is feasible. A prospective study to determine detection efficiency is now required of a consecutive routine pregnancy population in whom gestational age is determined by menstrual dates as is usually the case in clinical practice.