The Physician Shortage Area Program (PSAP) of Jefferson Medical College (Philadelphia, Pa) is one of a small number of medical school programs that addresses the shortage of rural primary care physicians. However, little is known regarding why these programs work. To identify factors independently predictive of rural primary care supply and retention and to determine which components of the PSAP lead to its outcomes. Retrospective cohort study. A total of 3414 Jefferson Medical College graduates from the classes of 1978-1993, including 220 PSAP graduates. Rural primary care practice and retention in 1999 as predicted by 19 previously collected variables. Twelve variables were available for all classes; 7 variables were collected only for 1978-1982 graduates. Freshman-year plan for family practice, being in the PSAP, having a National Health Service Corps scholarship, male sex, and taking an elective senior family practice rural preceptorship (the only factor not available at entrance to medical school) were independently predictive of physicians practicing rural primary care. For 1978-1982 graduates, growing up in a rural area was the only additionally collected independent predictor of rural primary care (odds ratio [OR], 4.0; 95% CI, 2.1-7.6; P<.001). Participation in the PSAP was the only independent predictive factor of retention for all classes (OR, 4.7; 95% CI, 2.0-11.2; P<.001). Among PSAP graduates, taking a senior rural preceptorship was independently predictive of rural primary care (OR, 2.5; 95% CI, 1.3-4.7; P =.004). However, non-PSAP graduates with 2 key selection characteristics of PSAP students (having grown up in a rural area and freshman-year plans for family practice) were 78% as likely as PSAP graduates to be rural primary care physicians, and 75% as likely to remain, suggesting that the admissions component of the PSAP is the most important reason for its success. In fact, few graduates without either of these factors were rural primary care physicians (1.8%). Medical educators and policy makers can have the greatest impact on the supply and retention of rural primary care physicians by developing programs to increase the number of medical school matriculants with background and career plans that make them most likely to pursue these career goals. Curricular experiences and other factors can further increase these outcomes, especially by supporting those already likely to become rural primary care physicians.