Background: Both B-type natriuretic peptide (BNP) and renal function are prognostic indicators of survival in patients with congestive heart failure (CHF). However, relationships between BNP, renal function, and heart failure as an emergency diagnosis are unknown. Methods: The Breathing Not Properly Multinational Study was a prospectively designed diagnostic test evaluation study conducted in seven centers. Of 1,586 participants who presented with acute dyspnea, 1,452 patients (91.6%) had both BNP level and baseline estimated glomerular filtration rate (eGFR) available. Patients with an eGFR less than 15 mL/min/1.73 m2 and those on dialysis therapy were excluded. The final diagnosis was adjudicated by two independent cardiologists who were blinded to BNP results. Results: The final diagnosis was CHF in 715 patients (49.2%). Raw and log-log transformed correlations between BNP and eGFR values were r = −0.19 and r = −0.17 for those with CHF and r = −0.20 and r = −0.31 for those without CHF (both P < 0.0001 for r ≠ 0). Mean BNP levels were 561.6 pg/mL (162.3 fmol/mL), 647.5 pg/mL (187.1 fmol/mL), 745.6 pg/mL (215.5 fmol/mL), and 850.7 pg/mL (245.8 fmol/mL) for those with CHF and 85.4 pg/mL (24.7 fmol/mL), 131.7 pg/mL (38.1 fmol/mL), 297.2 pg/mL (85.9 fmol/mL), and 285.0 pg/mL (82.3 fmol/mL) for those without CHF in eGFR categories of 90 or greater, 89 to 60, 59 to 30, and less than 30 mL/min/1.73 m2, respectively. The area under the receiver operating characteristic curve and optimum cut points for BNP were 0.91 and 70.7 pg/mL (20.4 fmol/mL), 0.90 and 104.3 pg/mL (30.1 fmol/mL), 0.81 and 201.2 pg/mL (58.1 fmol/mL), and 0.86 and 225.0 pg/mL (65.0 fmol/mL) for the eGFR categories of 90 or greater, 89 to 60, 59 to 30, and less than 30 mL/min/1.73 m2, respectively. Conclusion: Renal function correlates weakly with BNP and influences the optimal cut point for BNP, particularly in those with an eGFR less than 60 mL/min/1.73 m2. Am J Kidney Dis 41:571-579. © 2003 by the National Kidney Foundation, Inc.