Medicare Part B pays outpatient physicians according to the billed Current Procedural Terminology (CPT) codes, which differ in procedure and intensity. Since many performed services merely differ by intensity, physicians have an incentive to upcode services to increase profitability of a visit. Using nationally representative data from the 2001 to 2003 Medicare Current Beneficiary Survey, this paper explores the effect of Medicare Part B fee differentials on the upcoding of general office visits (i.e. for established patient visits with CPT codes of 99212-99215). It finds strong evidence that these fee differentials influence physician's coding choice for billing purposes across a variety of specialties. For general office visits, Medicare outlays attributable to upcoding may sum to as much as 15% of total expenditures for such visits. Medicare has much to gain financially by clarifying its classification rules. Until the distinctions between types of Medicare visits are redefined in a way that eliminates ambiguity, upcoding under Medicare Part B is likely to continue.