Serum samples for creatine kinase (CK) activity were obtained serially during 48 h in patients with acute myocardial infarction (AMI) diagnosed according to conventional criteria. Infarct size was estimated as peak enzyme value (CKmax), area under the enzyme curve measured planimetrically (AUC) and was also calculated according to Sobel et al. (IS) by a computer. In 16 patients with enzyme curves judged to fit the computer method, there was a good correlation between AUC and IS (r = 0.97), between AUC and CKmax (r = 0.99) and accordingly also between IS and CKmax (r = 0.96). The results from 34 consecutive patients with AMI were plotted in the graphs obtained from the 16 patients with "ideal" enzyme curves. 15% fell outside +/- 2 SD when AUC was correlated to IS, 3% when AUC was correlated to CKmax and 12% when IS was correlated to CKmax. There was no difference between enzyme curves falling outside or within +/- 2 SD. The explanation for the high correlation between these three methods may be that all these ways of estimating the high concentration between these three methods may be that all these ways of estimating infarct size are imprecise and only give a rough estimation of the size of the infarction.