The recent decline in the number of medical students choosing careers in the primary care specialties has engendered increasing concern that economic factors are becoming more important in influencing the career choices of medical students. In order to assess the relationship of first-year medical students' income expectations to whether they chose to specialize in family practice, the authors analyzed data from 532 graduates of Jefferson Medical College (classes of 1987-1989), using the Jefferson Longitudinal Study. At entrance to medical school, each student listed his or her initial specialty preference and future expected peak income; the determination of actual specialty choice was based on the first year of postgraduate training. Both expected peak incomes and freshman specialty choices were independent predictors of actual specialty choices. The students who entered family practice residencies had lower initial expected peak incomes than did the students entering other specialties, especially the surgery specialties. In addition, according to logistic regression analysis, the students with relatively lower income expectations and a freshman preference for family practice were predicted to be nine times more likely to enter family practice residencies than were students with higher income expectations and no initial family practice preference (56% versus 6%). This study suggests that a freshman's income expectation is an important predictor of family practice specialty choice, independent of age, sex, degree of indebtedness, and initial specialty preference. The authors discuss their results in light of the decline in the number of medical students choosing family practice and the other primary care specialties.