Blood enzyme activities that might be suggestive of organ-specific damage were measured over a six-week period in two, male, long-distance runners training for a marathon. Striking changes were observed in the CK and LD isoenzymes. Runner B exhibited a flipped LD1/LD2 isoenzyme ratio that paralleled his persistent MB CK elevation. The hepatic enzyme ALT was transiently elevated in Runner A. These data suggest that increases of cardiac isoenzymes (MB CK and LD1) and the specific hepatic enzyme ALT could lead to an inappropriate laboratory diagnosis of a myocardial infarction or liver disease, respectively, in a healthy runner during intense training for a marathon.