The clinical diagnosis of active lymphocytic myocarditis is often difficult, owing to the nonspecific nature of the clinical presentation. Although endomyocardial biopsy is potentially the best available tool for establishing this diagnosis, reliable and quantitative criteria for light microscopy are needed. In an attempt to define specific histologic criteria for the diagnosis of myocarditis, we counted the number of interstitial lymphocytes in 20 high-power (X400) microscopic fields in each of 170 endomyocardial biopsy specimens. The normal mean number of myocardial lymphocytes was determined to be fewer than 5.0 per high-power field, and mean values greater than this were considered to represent active lymphocytic myocarditis. With the use of these histologic criteria, the endomyocardial biopsy may be helpful in the detection of active myocarditis and in the evaluation of treatment.