OBJECTIVES: To assess obstetrician-gynecologists' judgments of gestational age of viability and earliest age of medical intervention for preterm delivery, and to associate these practice decisions with physician characteristics. METHODS: Questionnaires were mailed to 1193 members of the American College of Obstetricians and Gynecologists (ACOG). RESULTS: The response rate was 59%. The majority of respondents considered 24 weeks the earliest age a fetus is potentially viable (57%) and at which they would routinely perform cesarean section for fetal distress (58%). Those respondents who judged viability as 23 weeks or less were more likely to have been in practice for a shorter period (p < 0.05), be a maternal-fetal medicine specialist (p < 0.005), and be from southern or central states (p < 0.005). Similarly, those respondents who would not intervene for fetal distress until 26 weeks gestation were more likely to have been in practice for longer (p < 0.01), to have performed fewer deliveries (p < 0.05), to be in solo practice (p < 0.01), and not to be a maternal-fetal medicine specialist (p < 0.01); males and females did not differ when controlling for age (p = 0.552). CONCLUSION: Obstetrician-gynecologists' judgment of viability threshold is consistent with standard estimates of 24 weeks. Viability judgment and reported earliest age for routine intervention both differ by physician characteristics.