To test the hypothesis that patients with normal serum levels of creatine kinase (CK) but elevated percentages of MB isoenzyme fractions in suspected myocardial infarction may have sustained clinically significant events, we studied the hospital course of 347 consecutive patients admitted with suspected myocardial infarction. Two hundred twenty-three patients had normal CK levels (182 ± 44 IU) and normal MB percentages (normal group), 68 had elevated levels of both CK (1395 ± 178 IU) and MB percentage (10.5 ± 0.6) (macroinfarction group), and 40 had normal CK levels (96 ± 7 IU) but elevated MB percentages (9.6 ± 0.5) with typical enzyme curves (macroinfarction group). Compared to the normal group, macroinfarction patients were older, had more congestive heart failure, required more intensive monitoring and therapy during longer stays, and sustained a higher in-hospital mortality rate. Thus, these macroinfarction patients are at increased risk and therefore warrant aggressive treatment and further evaluation.