Background Inflammation associated with acute myocardial infarction (AMI) is frequently marked by a peripheral leukocytosis and relative neutrophilia. Whether this process may contribute to the development of postinfarction congestive heart failure (CHF) is not established. The objective of this study was to examine the association between hospital admission peripheral total leukocyte count and the neutrophil percentage and the subsequent development of CHF in patients with AMI. The study was designed as a retrospective cohort study in the setting of a tertiary referral hospital. Participants included 185 patients discharged with a diagnosis of AMI between May 1 and Sept 30, 1996. Methods and Results Outcome measures included clinical episodes of CHF with confirmatory chest roentgenogram findings and/or echocardiographic evidence of contractile dysfunction. Multivariable logistic regression analyses were performed to examine the relation between the total leukocyte count, neutrophil percentage, and the development of CHF in the first 4 days after AMI while controlling for baseline characteristics and early therapeutic interventions. Thirty-one percent of the cohort had a leukocyte count >11.0 ×10⁹/L on admission to the hospital; 65% had a neutrophil percentage >65%, and 61% had a lymphocyte percentage ≤25%. CHF developed in 43% of the cohort. Of these, 92.5% had relative neutrophilia (neutrophil percentage >65%) compared with 45% of those in whom CHF did not develop. Multivariable analysis revealed a highly significant association between relative neutrophilia and the subsequent development of CHF (odds ratio 14.3; 95% confidence interval 5.2 to 39.3). Conclusions Relative neutrophilia on admission to the hospital in patients with AMI is significantly associated with the early development of CHF. This association may help in the identification of individuals at high risk who might benefit from more aggressive interventions to prevent or reduce the risk of CHF. (Am Heart J 2000;139:94-100.)