The purpose of this study was to determine a simple and reliable procedure of estimating acute myocardial infarct (AMI) size by measuring serum creatine kinase MB (CK-MB) in few daily blood samples. In 13 patients with AMI blood samples were drawn every second hour for 60 h for determination of serum CK-MB activity. Infarct size was calculated using the CK-MB values of all samples and compared to the size calculated according to various models based on enzyme levels in few samples. Two models, using 3 daily samples, showed very high correlations and satisfactory standard errors of estimate when compared to the infarct size calculated from all samples. One of the 2 models was based on a computerized log-normal curve fit programme and one on accumulation of serum activities of CK-MB. The coefficient of variation of infarct size estimated from thrice-daily sampling was 7.4 and 9.4 for the 2 models. Considering the twenty-fold variation in infarct size a satisfactory quantitation is achieved from 3 daily samples.