In order to investigate the magnitude and kinetics of the C-reactive protein (CRP) response after differing degrees of tissue injury, we studied changes in serum concentration of this acute phase protein in 19 patients after mild or extensive acute myocardial infarction. An increase in serum CRP concentration was seen in all patients. The rate of increase in concentration was found to be exponential, with a mean hourly rate constant for the entire group of patients of 0.085 (doubling time, 8.2 h). Patients with extensive infarction attained mean serum CRP levels about 4 times as great as did patients with mild infarction. No difference could be shown in the mean rate constant between these groups, the greater CRP response in the former group resulting principally from a more protracted period of rise in serum CRP concentration. A lag period before serum CRP levels began to rise was noted in only 4 of the 13 patients in whom this could be assessed. 7 of 10 patients with presumed unstable angina (coronary insufficiency) showed no rise in CRP concentration, while a small increase as noted in 3 patients. The data suggest that acute tissue injury, such as myocardial infarction, rapidly leads to acceleration in synthesis of CRP, and that the duration of this period of acceleration is related to the extent of tissue injury.