A novel, highly sensitive and specific N-Terminal-proBNP (NT-proBNP) assay based on a sandwich format has been developed. The assay time is below 2 hours and no extraction process is needed. The calibration curve covers a NT-proBNP concentration range from 0 pmol/L up to 600 pmol/L. The analytical detection limit of the assay was estimated to be 2.7 pmol/L (3 SD). The intra-assay coefficient of variation is 5.7% (at 50 pmol/L) and 6.1% (at 250 pmol/L), while the inter-assay CVs are 15.8% (15 pmol/L) and 8.2% (250 pmol/L). There is no significant interference by bilirubin (up to 900 mumol/L), haemoglobin (up to 10 g/L), rheumatoid factors (up to 975 IU/mL), triglycerides (up to 20.5 mmol/L), biotin (up to 50 micrograms/L), digoxin (up to 100 micrograms/L) and digitoxin (up to 200 micrograms/L). The analyte NT-proBNP is fully stable in whole blood over 3 days and in EDTA-plasma over 24 hours. This good stability of NT-proBNP compared to other less stable natriuretic peptides is a significant advantage and a main prerequisite for a routine diagnostic marker. Preliminary results of using this new assay in clinical studies for diagnosing and monitoring left ventricular dysfunction demonstrate that there is a significant gain in diagnostic validity.