OBJECTIVES: Doctors’ practice and prescribing patterns are based on many factors, some of which are not observable.  We derived doctors’ prescribing patterns from U.S. claims data to show how it might be related with tumor necrosis factor (TNF) prescription decisions. METHODS: Based on U.S. claims data, we assigned doctors’ IDs based on the physician who treated the enrollee for the longest period of time after eliminating any emergency room, laboratory, and radiology services. Physician prescribing patterns were then calculated from J-codes from the outpatient service and prescription drug records for TNF and non-TNF biologics.      RESULTS: Among all TNF/anti-TNF prescribing doctors, patients who initiated their first TNF therapy were prescribed etanercept 42.8% of the time, adalimumab 31.2%, infliximab 21.1%, abatacept 1.7%, anakinra 0.5%, and rituximab 0.8% of the time.  If doctors’ practice/prescribing patterns favored TNF use or SubQ, patients were more likely to be switched to another TNF rather than to non-TNF biologics. CONCLUSIONS: Doctor’s prescribing patterns are important factors for prescription decisions. Any outcomes research models such as compliance, adherence or treatment effect studies should incorporate these patterns.   Models who fail to control for these variables might contain omitted variable bias.