The clinical value of maternal serum alpha-fetoprotein (AFP) as a guide to the outcome of threatened abortion was assessed. After the thirteenth week of gestation, abortion occurred more frequently (10/12) in women with abnormal serum AFP levels than in those (2/12) whose AFP concentrations were within the normal range. Low levels were present in women with blighted ovum and high concentrations were associated with intrauterine fetal death. In legal first and second trimester abortions, the circulating maternal AFP levels in postabortion samples were often higher than before abortion, irrespective of whether abortion was performed instrumentally or induced with prostaglandins. Maternal serum AFP levels provide a new means for prediction of the outcome of threatened abortion.