OBJECTIVE Our purpose was to study whether the in-hospital prognosis of anterior acute myocardial infarction (AMI) is influenced by preexistent collateral circulation to the infarct-related artery. BACKGROUND Collateral circulation exerts beneficial influences on the clinical course after AMI, but demonstration of improved survival is lacking. METHODS We studied 238 consecutive patients with anterior AMI treated by primary angioplasty within the first 6 h of the onset of symptoms. Fifty-eight patients with basal Thrombolysis in Myocardial Infarction (TIMI) flow >1 in the infarct-related artery or with inadequate documentation of collateral circulation were excluded. Collateral channels to the infarct-related artery before angioplasty were angiographically assessed, establishing two groups: 115 patients (64%) without collateral vessels (group A) and 65 patients (36%) with collateral vessels (group B). RESULTS There were no differences in baseline characteristics between groups A and B, except for the greater prevalence of previous angina in group B (15% vs. 34%, p = 0.003). During the hospital stay, 26 patients (23%) in group A and 5 (8%) in group B died (p = 0.01). Cardiogenic shock accounted for 74% of deaths. Cardiogenic shock developed in 30 patients (26%) in group A and in 4 (6%) in group B (p = 0.001). The absence of collateral circulation appeared to be an independent predictor of in-hospital death (odds ratio 3.4, 95% confidence interval 1.2 to 9.6, p = 0.02) and cardiogenic shock (odds ratio 5.6, 95% confidence interval 1.9 to 17, p = 0.002). CONCLUSIONS Preexistent collateral circulation decreases in-hospital death from anterior AMI by reducing the incidence of cardiogenic shock.