Both biologically active BNP1-32 and inactive NT-proBNP1-76 are released in circulation from prohormone, proBNP1-108. Recent data suggest that proBNP is detected in blood of heart failure (HF) patients. Blood levels, kinetics as well as determinants of proBNP1-108 secretion are unknown as compared to processed forms. We sought to measure and to compare blood levels of BNP and proBNP in HF and non HF patients. Methods Plasma natriuretic peptides were assessed in 131 HF patients and in 36 non-HF patients using both Triage BNP assay and BioRad proBNP assay. BioRad assay is highly specific of proBNP when the Triage BNP assay may be influenced by proBNP. Among HF patients, 33 were admitted because of acute HF and blood samples were analyzed on admission and at discharge. In order to compare BNP and proBNP levels, results are expressed as nmole/l. Results ProBNP was detected in all patients (104 ± 145 nmol/l) and demonstrated a high degree of correlation with BNP in non-HF patients (r = 0.95 [95% IC 0.90 – 0.97], p<0.0001) as well as in HF patients (r = 0.97, [0.95 - 0.98], p<0.0001). The proBNP/BNP ratio was higher in HF patients as compared to non-HF patients (0.82 ± 0.23 versus 0.62 ± 0.18, p < 0.0001). In patients with decompensated HF, proBNP/BNP ratio were similar on admission and at discharge, and changes in both BNP and proBNP were similar (0.91, [0.82-0.95], p < 0.0001). Interestingly, proBNP/BNP ratio was lower in the most severe HF patients according to discharge BNP and proBNP/BNP ratio was negatively correlated with discharge BNP (r = -0.40, [-0.65 – -0.06], p 0.02). Conclusion The intact precursor – proBNP- to both BNP and NT-proBNP circulates in both HF and non-HF patients and is an important part of the pool of circulating natriuretic peptides. HF seems to be characterized by a decrease in proBNP processing but, unexpectedely, at a lesser level in acute and/or severe HF as compared with less severe HF. Biological and physiological meanings of our results deserves further studies.