The aim of this study was to establish quantitative diagnostic criteria for lymphocytic myocarditis on autopsy samples by using a stereological cell profile counting method. We quantified and compared the presence of lymphocytes and macrophages in myocardial autopsy specimens from 112 deceased individuals who had been diagnosed with myocarditis according to the Dallas criteria and 86 control subjects with morphologically normal hearts.We found the mean number to be 52.7lymphocyte profiles/mm² (range 3.7-946; standard deviation 131) in the myocarditis group and 9.7 (range 2.1-25.9; standard deviation 4.6) in the control group. The cut-off value for the diagnosis of myocarditis was determined by calculating sensitivity plus specificity, which reached the highest combination at 13lymphocyte profiles/mm² (sensitivity 68%; specificity 83%). A considerable proportion of subjects in both the myocarditis and control groups had lymphocyte profile counts below 30/mm², representing a diagnostic challenge due to the increased risk of creating false negative or false positive results. We found it practically impossible to obtain a reliable macrophage count.The present data add new important information on lymphocyte counts in inflamed and non-inflamed myocardium. We suggest a cut-off value in the range of 11-16lymphocyte profiles/mm² for a reliable diagnosis of lymphocytic myocarditis from autopsy samples. To evaluate small inflammatory changes at low lymphocyte counts, a multidisciplinary approach should be implemented, in which diagnostic tools are used ancillary to histological examination. We advise against semi-quantification of macrophages based on cell profile counting.