Elevated creatine phosphokinase (CPK) MB isoenzyme has become accepted as a highly specific criterion for the diagnosis of myocardial infarction (MI). A patient with metastatic neuroendocrine carcinoma of the colon who had marked and persistent elevation of CPK-MB isoenzyme, in the absence of clinical and cardiographic evidence for MI, is described. The CPK-MB level was 25 percent (normal, <3 percent) on admission, 39 percent postoperatively, and 57 percent on discharge. A prompt decline in serum CPK-MB activity (11 percent, <3 percent) paralleled chemotherapy-induced tumor regression. Resurgence of the isoenzyme heralded recurrent disease. These findings suggest that CPK-MB may be a valuable adjunct marker in the diagnosis and monitoring of neuroendocrine carcinomas.