To provide a basis for international discussion of ethical problems, we studied responses of medical geneticists in 18 countries to questionnaires about 14 clinical cases and five screening situations. Of 1,053 asked to participate, 677 (64%) responded. There was greater than or equal to 75% consensus on five cases involving (1) disclosure of (1) conflicting diagnostic findings, (2) disclosure of ambiguous results, (3) disclosure of controversial interpretations, (4) protection of mother's confidentiality in cases of false paternity, and (5) nondirective counseling about 45,X and XYY syndrome. A majority (51%-60%) would disclose the diagnosis to relatives at risk for Huntington disease or hemophilia A, against the patient's wishes; would disclose which parent carries a translocation causing Down syndrome; and would disclose XY genotype in a female. As reproductive options for patients with disorders not diagnosable prenatally, 84% would discuss artificial insemination by a donor, 66% would discuss in vitro fertilization with donor egg, and 46% would discuss surrogate motherhood. In all, 85% would perform prenatal diagnosis for (or would refer) parents who refuse abortion, 75% for maternal anxiety, and 42% for selection of fetal sex. Screening questions showed that 72% believed that workplace screening should be voluntary and that results should be confidential.