The purpose of this report was to evaluate the reproducibility and harmonization of cardiac marker tests and to describe the current situation concerning quality of assays for cardiac markers on the basis of the results of the external quality control schemes (EQAS) of Labquality Ltd., Helsinki, Finland in the period 2002 to 2005. Finnish EQAS surveys obtained for proficiency samples at low marker concentration indicated that the overall coefficient of variation (CV) between laboratories for CK-MBmass and troponin I exceeded 10 %, while for cardiac troponin T the CV was 8.6 %. Intra-laboratory reproducibility was investigated in a single laboratory using concomitant testing in the same EDTA plasma samples to establish cut-off limits for one CK-MBmass and three troponin assays. The 10 % imprecision limit obtained from the concomitant testing in the same samples for CK-MBmass was (by Elecsys) 8.5 microg/L, for cardiac troponin T (by Elecsys) 0.023 microg/L and for cardiac troponin I (by AxSYM) and by Immulite 2000) 0.85 microg/L and 0.63 microg/L. At present, it is recommended that laboratories determine the concentration at which the 10 % imprecision for a specific cardiac marker assay is reached, because the assays generally do not reach that imprecision at the level of the 99th percentile value, usually taken as decisional level. However, common efforts of scientific societies and professional diagnostic industry associations internationally are needed if consensus is to be reached on standardization of immunoassays for cardiac markers and uniform results obtained among laboratories.