This study examines the degree to which patient characteristics predict physicians' ad hoc decisions regarding acceptance of Medicare assignment. The study is based on a random sample of Medicare Part B enrollees living independently in the Salem, Oregon metropolitan area. Beneficiary characteristics and beneficiary reports of physician behavior are obtained from an hour long face-to-face survey. The findings show that patient characteristics are significant predictors of physician behavior. Those respondents with poor health, no supplementary coverage to Medicare Part B, and who are more sensitive to the cost of health care are significantly more likely to report that their physician accepts assignment than are respondents without these characteristics. Policy and research implications are discussed.