Carrier testing for Tay-Sachs disease should be offered to couples when at least one individual is of Ashkenazi Jewish (carrier frequency 1/30), Pennsylvania Dutch, Southern Louisiana Cajun, or Eastern Quebec French Canadian descent. Ideally, testing is done prior to conception. For Ashkenazi Jews, in whom DNA testing identifies 99.9% of carriers, DNA testing is the preferred method to ascertain carriers . For non-Jewish individuals seeking carrier testing, enzyme assay should be done initially and positive or indeterminate results should be confirmed by DNA mutation analysis. If only one partner is descended from a high-risk group, that person should be tested first; only if he/ she is a carrier should the other partner be tested. If the couple is pregnant at the time carrier testing is requested, both partners should have enzyme testing (leukocyte assay for the pregnant woman and serum assay for the father) and DNA testing sent concomitantly to expedite counseling and action. Carriers are individuals with a disease causing DNA mutation or carrier range enzyme analysis results on both serum and leukocytes with no detectable mutation and no pseudodeficiency alleles. Noncarriers are individuals with normal enzyme results or carrier range enzyme results and a pseudodeficiency allele on DNA mutation analysis. If both partners are found to be carriers they should be counseled of a 25% risk of having an affected child with each pregnancy. Options to modify this risk include prenatal diagnosis by amniocentesis or chorionic villus sampling, egg or sperm donation, preimplantation diagnosis or adoption.