Proficiency testing (PT) is a mandated requirement for clinical laboratories doing therapeutic drug monitoring and is an invaluable tool to help laboratories identify and correct problems in analytical procedures. The AIDS Clinical Trial Group Pharmacology Quality Assurance Committee implemented a new antiretroviral PT program for all currently available antiretroviral drugs in 2001. The PT program was designed for the AIDS Clinical Trial Group Pharmacology Specialty Laboratories actively involved in assaying these drugs in clinical trial samples so as to comply with the Clinical Laboratory Improvement Amendments. Results from the first 3 rounds of PT have been analyzed and reported and provided support for formalizing the guidelines of the PT testing program. The PT program has expanded with the addition of nucleoside reverse transcription inhibitors (NRTIs) and atazanavir. This report includes results from rounds 4 to 7 over 2 additional years of standard operations. Additionally we include results from NRTIs for all rounds and atazanavir for a single round. There were 9 participating laboratories. Eight used high-performance liquid chromatography as the primary method of detection and 2/8 also reported LC-MS-MS results. One laboratory used LC-MS-MS as their primary detection method. All laboratories measured protease inhibitors, most measured at least 1 non-nucleoside reverse transcription inhibitor and 5 had NRTI capabilities. Results were normally distributed and the acceptance range of +/-20% best corresponded to a 95% confidence interval. Overall score for 9 participating laboratories was 96% correct out of 1826 challenges over 4 rounds. Laboratories scored 95, 98 and 97% correct for protease inhibitors, non-nucleoside reverse transcription inhibitorss and NRTIs, respectively. Three laboratories reporting LC-MS-MS results had 92% correct (347/378) challenges for all drugs. The percentage of correct results is about the same as previously reported. There is a continued need for a PT program to help participating laboratories maintain essential quality assurance and quality control.