Elevated serum creatine kinase MB isoenzyme (CK-MB) levels are regarded as a sensitive and specific marker for myocardial injury. Serum CK-MB was measured in male marathon runners during training and after competition. Mean serum CK-MB measured by a quantitative electrophoretic technique (normal, less than 5 IU/L) showed borderline elevation during training with peaks 24 hours after competition. Mean CK-MB in 64 serum samples from 35 runners after competition was 130 IU/L or 8.3% of total CK activity. Levels of CK-MB averaging 26 times the upper limits of normal would usually be considered indicative of massive myocardial necrosis. Myocardial scintigraphy with technetium Tc 99m pyrophosphate was performed after competition in 12 randomly selected runners with a mean postrace serum CK-MB level of 160 IU/L or 13%. Infarct-avid ("hot-spot") scintigraphy, appropriately timed to detect underlying myocardial abnormalities, was within normal limits in all subjects. Normal results of infarct-avid scintigraphy coincident with marked serum CK-MB elevations strongly suggests that CK-MB arises from a noncardiac or skeletal muscle source in these runners.