Objectives The purpose of this study was to investigate circulating pro–B-type natriuretic peptide (proBNP 1–108) in the general community and evaluate its ability to detect left ventricular (LV) dysfunction. Background The current concept for cardiac endocrine function is that, in response to cardiac stress, the heart secretes B-type natriuretic peptide (BNP 1–32) and amino-terminal pro–B-type natriuretic peptide (NT-proBNP 1–76) after intracardiac cleavage of their molecular precursor, proBNP 1–108. We hypothesized that proBNP 1–108 circulates in normal human subjects and that it is a useful biomarker for LV dysfunction. Methods Our population-based study included a cohort of 1,939 adults (age ≥45 years) from Olmsted County, Minnesota, with 672 participants defined as healthy. Subjects underwent in-depth clinical characterization, detailed echocardiography, and measurement of proBNP 1–108. Independent factors associated with proBNP 1–108 and test characteristics for the detection of LV dysfunction were determined. Results ProBNP 1–108 in normal humans was strongly influenced by sex, age, heart rate, and body mass index. The median concentration was 20 ng/l with a mean proBNP 1–108 to NT-proBNP 1–76 ratio of 0.366, which decreased with heart failure stage. ProBNP 1–108 was a sensitive (78.8%) and specific (86.1%) biomarker for detecting LV systolic dysfunction, which was comparable to BNP 1–32, but less than NT-proBNP 1–76, in several subsets of the population. Conclusions ProBNP 1–108 circulates in the majority of healthy humans in the general population and is a sensitive and specific biomarker for the detection of systolic dysfunction. The proBNP 1–108 to NT-proBNP 1–76 ratio may provide insights into altered proBNP 1–108 processing during heart failure progression. Thus, this highly specific assay for proBNP 1–108 provides important new insights into the biology of the BNP system.