Congress enacted the Clinical Laboratory Improvement Amendments of 1988 (CLIA) to promote uniform quality and standards among all testing sites in the United States. The performance indicators specified in the legislation are proficiency testing (PT) performance and periodic inspections.To evaluate variation in PT performance by type of testing facility during the first year of compulsory participation under CLIA.All 1994 PT score data electronically reported to the Health Care Financing Administration as a component of compliance with the CLIA regulations were obtained. Over 1.2 million PT event scores from 17058 unique testing sites were sorted into 2 groups based on the type of testing facility: hospitals and independent laboratories (HI) and all other testing sites (AOT).Satisfactory and unsatisfactory performance rates for HI and AOT for each analyte and/or test, according to the criteria specified by the CLIA regulations.The aggregate rates of satisfactory event performance for all regulated analytes, tests, and specialties were 97% and 91% for the HI and AOTgroups, respectively. The aggregate odds ratio for unsatisfactory PT event performance for the AOT group compared with the HI group was 2.89, with a range of 2.19 to 7.51 for the individual analytes.There was a consistent difference in PT performance during the first full year of compulsory PT under the CLIA regulations based on the type of testing facility performing the analysis. Traditional testing sites achieved higher rates of satisfactory performance than newly regulated, alternative testing sites.