We analyzed referral rates by patient characteristics, physician characteristics, and visit setting. Variables analyzed included age (0-3, >3-18, >18-45, >45-65, and >65 years), sex, race (white, black, and other by patient report), insurance (private, Medicare, Medicaid, and other/uninsured [included worker's compensation, self-pay, charity, other, and unknown insurance]), and US region (Northeast, Midwest, South, and West). In 2005, the survey item for patient insurance type changed from a “mark one” form of payment to “mark all that apply,” potentially affecting any analysis of trends that rely on patient insurance type during this period. In addition, in the NHAMCS data, the referral disposition survey item (our main outcome) changed from “referred to other physician/clinic” to “referred to other physician” in 2001. Physician characteristics included whether physicians practiced in a solo setting, owned their practice in part or in full, made consults with patients by e-mail or telephone (first collected in 2001), had any form of electronic medical record, and received more than 50% of their income from managed care contracts or Medicaid (first collected in 2003).