Background: There is an increasing awareness of the need for external quality control of diagnostic virology. Objectives: To assess the quality of nucleic acid amplification tests (NAT) of herpes simplex within Europe. Study design: Herpes simplex virus (HSV) proficiency panels were produced at the Swedish Institute for Infectious Disease Control on behalf of the European Union Concerted Action for Quality Control of Nucleic Acid Amplification in 1999 and 2000. Nine reference laboratories evaluated the production process. Each panel consisted of 12 coded samples with various concentrations of inactivated, freeze-dried HSV type 1 (HSV-1), and HSV type 2 (HSV-2), or negative controls. Positive samples included HSV-1 and HSV-2 in a range of concentrations (2×10² to 2×10⁷ genome copies per ml) similar to those found in cerebrospinal fluids from patients with HSV encephalitis. Results: Sixty-six participants reported a total of 76 data sets for panel 1, and 71 reported 78 data sets for panel 2. The majority of the participants employed qualitative ‘in-house’ polymerase chain reaction (PCR) methods, either in a single, nested or semi-nested format. For panel 2, 9 laboratories reported use of ‘real-time’ PCR in contrast to 3 for panel 1. Three laboratories submitted quantitative results on both panels. Thirty percent of the data sets had correct results for the entire panel 1. In 6 data sets (8%) a total of 11 false positive results were reported. For panel 2, 28% of the data sets had correct result. Nineteen false positive results were reported in 14 data sets (18%), but most of the incorrect results reflected a lack of test sensitivity. Conclusions: The relatively high frequency of false positive results and the large number of false-negative results, albeit at low copy number, stress the need for improvement in the quality of HSV NAT and for external quality control programmes.