Background: Preoperative B-type natriuretic peptide (BNP) is known to predict adverse outcomes after cardiac surgery. The value of postoperative BNP for predicting adverse outcomes is less well delineated. The authors hypothesized that peak postoperative plasma BNP (measured postoperative days 1-5) predicts hospital length of stay (HLOS) and mortality in patients undergoing primary coronary artery bypass grafting, even after adjusting for preoperative BNP and perioperative clinical risk factors. METHODS:: This study is a prospective longitudinal study of 1,183 patients undergoing primary coronary artery bypass grafting surgery. Mortality was defined as all-cause death within 5 yr after surgery. Cox proportional hazards analyses were conducted to separately evaluate the associations between peak postoperative BNP and HLOS and mortality. Multivariable adjustments were made for patient demographics, preoperative BNP concentration, and clinical risk factors. BNP measurements were log10 transformed before analysis. Results:One hundred fifteen deaths (9.7%) occurred in the cohort (mean follow-up = 4.3 yr, range = 2.38-5.0 yr). After multivariable adjustment for preoperative BNP and clinical covariates, peak postoperative BNP predicted HLOS (hazard ratio [HR] = 1.28, 95% CI = 1.002-1.64, P = 0.049) but not mortality (HR = 1.62, CI = 0.71-3.68, P = 0.25), whereas preoperative BNP independently predicted HLOS (HR = 1.09, CI = 1.01-1.18, P = 0.03) and approached being an independent predictor of mortality (HR = 1.36, CI = 0.96-1.94, P = 0.08). When preoperative and peak postoperative BNP were separately adjusted for within the clinical multivariable models, each independently predicted HLOS (preoperative BNP HR = 1.13, CI = 1.05-1.21, P = 0.0007; peak postoperative BNP HR = 1.44, CI = 1.15-1.81, P = 0.001) and mortality (preoperative BNP HR = 1.50, CI = 1.09-2.07, P = 0.01; peak postoperative BNP HR = 2.29, CI = 1.11-4.73, P = 0.02). Conclusions: Preoperative BNP may be better than peak postoperative BNP for predicting HLOS and longer term mortality after primary coronary artery bypass grafting surgery.