OBJECTIVE: HIV treatment has involved infectious disease specialists and other specialties. We studied the relative use of phenotypic resistance testing among specialties and by different regions in the U.S over time. METHODS: Phenotypic resistance testing data from Virologic was aggregated for 7/01 ¨C 6/03. Tests were assigned to each ordering physicians. Self-reported specialty was identified and an automated sample generated. Specialties were grouped as infectious disease, internal medicine, primary care (family practice & general practice) and all others. "Unspecified" physician records were deleted. Data were analyzed for statistically significant differences in: the average use of resistance testing by each specialty in July 2001 - June 2002 compared to July 2002 - June 2003 across the US, and the use of resistance testing in the each US census region compared to the average US-wide use for each respective specialty. RESULTS: There were approximately 2600 physicians in the sample for which 250,000 drug resistance tests were conducted. It showed across the US the average use of phenotypic resistance testing by Infectious Disease statistically increased 5% (p = 0.05) for July 2002 - June 2003 compared to July 2001 - June 2002 and the use by other specialties decreased. Within each specialty there were statistically significant differences by geographic region in the use of phenotypic resistance testing. CONCLUSION: Based on its relative ease of interpretation compared to genotypic information we might have predicted that the use of phenotypic resistance testing would increase for non-Infectious Disease specialists. The reverse was true i.e. it increased for Infectious Disease relative to other specialties use. Significant differences in the use of phenotypic resistance testing were also seen by regions. Contributing factors may include increasing adoption of phenotypic resistance testing by Infectious Disease specialists, variation in patient severity and financial constraints.