We report the development of a rapid nonradioactive technique for the genetic prediction of human disease and its diagnostic application to hemophilia A. This method is based on enzymatic amplification of short segments of human genes associated with inherited disorders. A novel feature of the procedure is the use of a heat-stable DNA polymerase, which allows the repeated rounds of DNA synthesis to proceed at 63 degrees C. The high sequence specificity of the amplification reaction at this elevated temperature permits restriction-site polymorphisms, contained in the amplified samples, to be analyzed by visual inspection of their digestion products on polyacrylamide gels. By means of this method, we have performed carrier detection and prenatal diagnosis of hemophilia in two families with use of the factor VIII intragenic polymorphisms identified by the restriction enzymes BclI and XbaI. Predictions can be made directly from chorionic villi, without previous DNA extraction, and fetal sex can be determined by amplification of sequences specific for the Y chromosome. Specific amplification of genomic sequences with heat-stable DNA polymerase is applicable to the diagnosis of a wide variety of inherited disorders. These include diseases diagnosed by restriction-site variation, such as Duchenne's muscular dystrophy and sickle cell anemia, those due to a collection of known mutations, such as beta-thalassemia, and those due to gene deletion, such as alpha-thalassemia.