We estimated infarct size in 95 patients with clinically uncomplicated infarction from total CK and results were compared to infarct size index based on plasma MB CK. Among patients with relatively low infarct size index indicative of relatively small infarctions (less than or equal to 20), estimates with total CK correlated closely with those based on MB CK (r = 0.88). In contrast, among patients with larger infarcts (infarct size index greater than 20) the correlation was poor. In this group, values for infarct size index based on total CK were 67 +/- 4 (SE)% greater than those based on MB CK. In the overall population, infarct size index based on MB (14 +/- 1 (SE) CK-gram-equivalents/m2) was less than that based on total CK (21 +/- 2). These disparities appear to reflect occult release of non-cardiac CK in patients with more extensive infarction. Thus, even in patients without overt clinical complications enzymatic estimates of infarct size are more accurate when based on MB rather than total plasma CK.