A growing scientific literature highlights concern about the influence of social bias in medical care. Differential treatment of male and female patients has been among the documented concerns. Yet, little is known about the extent to which differential treatment of male and female patients reflects the influence of social bias or of more acceptable factors, such as different patient preferences or different anticipated outcomes of care. This paper attempts to ascertain the underlying basis for an observed differential in physicians' tendency to advice activity restrictions for male and female patients. We explore the extent to which the gender-based treatment differential is attributable to: (1) patients' health profile, (2) patients' role responsibilities, (3) patients' illness behaviors, and (4) physician characteristics. These four categories of variables correspond to four prominent social science hypotheses concerning gender differences in health and health care utilization (i.e, biological basis hypothesis, fixed role hypothesis, socialization hypothesis, physician bias hypothesis). Data are drawn from the Medical Outcomes Study (MOS), a longitudinal observational study of 1546 patients of 349 physicians practicing in three U.S. cities. Multivariate logistic regression is used to evaluate the likelihood of physician-prescribed activity restrictions for male and female patients, and to explore the absolute and relative influence of patient and physician factors on the observed treatment differential. Results reveal that the odds of prescribed activity restrictions are 3.6 times higher for female patients than for males with equivalent characteristics. The observed differential is not explained by differences in male and female patients' health or role responsibilities. Gender differences in illness behavior and physician gender biases both appear to contribute to the observed differential. Female patients exhibit more illness behavior than males, and these behaviors increase physicians' tendency to prescribe activity restrictions. After accounting for illness behavior differences and all other factors, the odds of prescribed activity restrictions among female patients of male physicians is four times that of equivalent male patients of those physicians. Medical practice, education, and research must strive to identify and remove the likely unconscious role of social bias in medical decision making.