To determine whether differences in the kinetics of isoforms of MM and MB creatine kinase affect their ability to detect coronary patency in patients treated with thrombolytic agents, we compared MM and MB isoform profiles in 33 consecutive patients. Results were discordant in 13 of the 33 at 1 hour. When the rates of increase of both isoforms were considered, discordance was present in only 10 of the 33 patients. In five patients %MM3 rose rapidly during the second hour and infarct-related vessels were patent. Four of the five without a rapid increase had occluded infarct-related vessels. These data suggest that criteria based on rates of change in %MB2 are more sensitive than those based on %MM3. However, criteria based on %MM3 are more likely to identify patients in need of interventions to maintain coronary patency.