OBJECTIVE: Our purpose was to compare the use of the Iowa Expanded Serum Screening program (maternal serum α-fetoprotein, human chorionic gonadotropin, and unconjugated estriol) by obstetricians and family physicians. STUDY DESIGN: A registry was used to identify all obstetricians (160) and family physicians (404) who practice obstetrics in Iowa. A questionnaire exploring attitudes and practice patterns related to serum screening was mailed to these physicians. RESULTS: The response rate was 80.3% overall. There were significant differences in the offering of serum screening (p < 0.001) for following three responses: obstetricians were more likely to offer screening through the Iowa Expanded Serum Screening Program than were family physicians (89.3% vs 57.2%) and less likely to offer screening outside the state program (9.9% vs 39.0%), whereas family physicians were more likely to not offer screening (0.8% vs 3.8%). When the responses of the physicians who offer screening were analyzed, several differences emerged: (1) obstetricians were more likely than family physicians to offer screening to all obstetric patients (99.2% vs 92.1%, p < 0.05), whereas family physicians were more likely to offer testing to patients with perceived risks for neural tube defects or chromosomal abnormalities and to those patients who requested it, (2) obstetricians were more likely to recommend screening than family physicians (39.0% vs 25.7%, p < 0.05), who more often discouraged testing, although the latter did not reach statistical significance, (3) family physicians more than obstetricians felt that screening was not necessary if the patient would not terminate the pregnancy for neural tube defects (41.6% vs 19.1%, p < 0.001) or chromosomal abnormalities (39.7% vs 21.4%, p < 0.01), and (4) obstetricians used nurses to counsel patients to a much greater extent than family physicians did (44.6% vs 14.0%, p < 0.001). CONCLUSION: There is a significant difference in practice patterns between obstetricians and family physicians in their reported use and presentation of maternal serum screening. Guidelines based on outcome studies should be developed and followed by obstetricians and family physicians. (AM J OBSTET GYNECOL 1996;174:1361-5.)