PURPOSE. To compare the percentage of students who maintain interest in specializing in obstetrics-gynecology during medical school with the percentages of students maintaining interest in other selected specialties, and to examine changes of interest from obstetrics-gynecology to other specialties and from other specialties to obstetrics-gynecology. METHOD. A longitudinal cohort study comparing the stabilities of students' interests in obstetrics-gynecology and in other specialties was performed by using data on 3,889 graduates of 18 classes of Jefferson Medical College of Thomas Jefferson University between 1975 and 1992. RESULTS. The percentage of students who maintained interest in obstetrics-gynecology, as measured at the beginning and end of medical school, was 19%, compared with 40% for internal medicine and surgery, 39% for family medicine, and 22% for pediatrics. By the time they graduated, some students who had planned as freshman to pursue obstetrics-gynecology had changed their interests to internal medicine (19%), surgery (17%), family medicine (8%), or pediatrics (7%). In turn, obstetrics-gynecology attracted students who had initially expressed interest in other specialties: 17% from family medicine, 14% from surgery, 12% from internal medicine, and 8% from pediatrics. Despite the low percentage of students who maintained interest in obstetrics-gynecology, the overall percentage of students interested in obstetrics-gynecology at the time of graduation was somewhat greater than the percentage of students interested at the start of medical school. CONCLUSION. That only about one-fifth of the students initially interested in obstetrics-gynecology maintained their interest, and that many students' interests changed from one specialty to another, suggest that factors contributing to changes in interest need further investigation.