Between 20 and 30 per cent of all breast cancers show an overexpression of the growth factor HER2 and this implies a worse prognosis. Metastasising breast cancer that overexpress HER2 can effectively be treated with trastuzumab (Herceptin). In order to determine whether this treatment is indicated, HER2 expression must be quantified with immunohistochemical or molecular biological methods. Several factors may influence the laboratory analysis of HER2, such as choice of antibodies and probes, the reliability of the test performance itself and the performance of the pathologist(s) interpreting the test. National guidelines for HER2 testing have been published in several countries. In this article a series of quality assurance projects leading to the formulation of Swedish guidelines for HER2 testing is described. These projects have demonstrated that several Swedish pathology laboratories can reliably perform and interpret analysis of HER2 status today.