The protective effect of ischemic preconditioning has been confirmed in animal models. In this study we analyzed the clinical data of 110 cases of acute myocardial infarction (77 male, 33 female). Sixty-nine cases (group A) had ischemic manifestations prior to myocardial infarction, while forty one (group B) did not have. Our data showed that the clinical features in group B were quite different as compared with those in group A: (1) larger infarct size (ratio between necrotic size and ischemic size: 71.55 +/- 3.70 to 41.65 +/- 3.96, P < 0.0001); (2) higher peak level of serum cardiac enzymes (CPK: 2085.78 +/- 265.57 to 1329.80 +/- 189.44, P < 0.01; CK-MB: 102.73 +/- 12.47 to 47.38 +/- 8.83, P < 0.01); (3) poorer cardiac function [LVEF < 0.45: 12/41 (29.3%) to 6/69 (8.7%), P < 0.05]; (4) higher incidence of left ventricular aneurysm [9/41 (22.0%) to 3/69 (4.3%), P < 0.05] and (5) higher mortality rate in 4 weeks [6/41 (14.6%) to 2/69 (2.9%), P < 0.05]. The difference between the two groups is statistically significant. In addition, the effect of the duration of preconditioning on protection of myocardium, the potential mechanism of preconditioning and its clinical significance were discussed.