Background: The in vitro stability of a biomarker can determine whether it should be used in clinical practice where long delays between sampling and assay are common. We measured the in vitro stability of five novel biomarkers that are being evaluated for their diagnostic and/or prognostic utility in patients with heart failure: mid-regional pro-atrial natriuretic peptide (MR-proANP), mid-regional pro-adreno-medullin (MR-proADM), C-terminal pro-endothelin-1 (CT-proET-1), C-terminal pro-arginine vasopressin (copeptin) and ultrasensitive procalcitonin (PCT). Methods: Peripheral venous blood samples were obtained from 19 patients with chronic heart failure into four EDTA tubes. The first tubes were centrifuged immediately at 4°C with the other tubes stored at 20°C for 4, 24 or 72 hours (h) before centrifuging. Supernatant plasma was frozen and stored at −80°C until assay. The levels of analyte in samples processed with and without delay were compared using correlation analysis, paired t-tests and Bland-Altman plots. Results: Copeptin and PCT were stable up to 72 h at 20°C in whole blood and MR-proANP and MR-proADM up to 24 h. However, CT-proET-1 showed some signs of degradation after only 4 h with 94% of analyte recovered after 24 h, dropping to 80% after 72 h. Conclusions: MR-proANP, MR-proADM, copeptin and PCT are stable biomarkers and therefore suitable for introduction into routine clinical practice in a primary or secondary care setting where delays in sample preparation and assay are likely. Ideally, samples for measurement of CT-proET-1 should be centrifuged soon after venepuncture but the analyte is stable enough for most routine clinical purposes.