To examine the clinical course of patients with acute myocardial infarction complicated by "extension", we studied prospectively 141 patients who had been diagnosed as having acute myocardial infarction. The serum CKMB level of these patients was determined at 8-h intervals during the first 5 days following admission. The patients were classified into 3 groups. Group A (early extension): patients who showed CKMB re-elevation before the CKMB values reached normal levels (28%). Group B (late extension): patients who showed CKMB re-elevation after the normalization of serum CKMB levels (21%). Group C (control group): patients without CKMB re-elevation (51%). Patients in group A showed the most unfavourable clinical course with a greater rate of haemodynamic deterioration compared with patients in the B or C groups, and a higher rate of recurrent ischemic pain. We found no significant differences in these parameters between the B and C groups. We were unable to find any risk factor associated with the development of extension. The pattern of the serum CKMB curve may allow a separation of two different subgroups of patients with acute myocardial infarct extension: patients with early extension, who show a high prevalence of haemodynamic deterioration, and patients with late extension, characterized by small infarcts and a benign clinical course.