In a consecutive series of 783 patients with acute myocardial infarction, 13 (1.7%) suffered a stroke. In all but one case the strokes occurred among the 255 patients whose peak creatine kinase (CK) concentrations fell in the upper third of the range of values (over 1160 IU/l, about eight times the upper limit of normal); the exception was a patient with a pre-existing ventricular aneurysm. The incidence of stroke in the patients with CK over 1160 IU/l was 4.7%, 24 times the incidence when peak CK was below this value (0.2%). Higher peak serum enzyme concentrations were associated with an even higher incidence of stroke. Comparison of peak enzyme concentrations with cumulated CK showed a close correlation (r = 0.90 with peak CK; r = 0.85 with peak aspartate transaminase), suggesting that the peak enzyme values reflected infarct size. Thus the risk of stroke after infarction was a function of the size of the myocardial infarct; two-thirds of the patients had negligible risk of stroke and did not need anticoagulant prophylaxis.