OBJECTIVE To assess the association of preoperative brain natriuretic peptide with atrial arrhythmias and length of stay after cardiac surgery. METHODS A retrospective data analysis. METHODS All data were collected from patients who underwent cardiac surgery at a single institution, an academic hospital, between 2005 and 2010. METHODS Patient data were collected from the authors' institution's Perioperative Health Documentation System of cardiac surgeries. METHODS None. RESULTS The major findings were that individuals with a 10-pg/mL increase in brain natriuretic peptide were 1.005 (95% CI: 1.002, 1.009) times more likely to have an atrial arrhythmia than those with no increase in brain natriuretic peptide. A brain natriuretic peptide value ≥306 pg/mL was calculated to best predict an atrial arrhythmia. Those above the threshold were 1.455 (95% CI: 1.087, 1.947) times more likely to have an atrial arrhythmia than those below the threshold. Individuals above the threshold had a median of 3 days in the intensive care unit compared to 1 day for those below the threshold, as well as median hospital stays of 11 days for those below the threshold. CONCLUSIONS The results indicated that elevated brain natriuretic peptide was associated with increased risk of atrial arrhythmias and prolonged length of hospital stay after cardiac surgery. Identifying at-risk patients is important to guide preventative strategies for postoperative atrial arrhythmias.