BACKGROUND: The usefulness of brain-natriuretic-peptide (BNP) and N-terminal-pro-brain-natriuretic-peptide (NT-proBNP) for monitoring of chronic heart failure (CHF) patients has been questioned because of high levels of unexplained variation. AIMS: Week-to-week total variance (CV(T)), unexplained variation (CV(I)), reference change values (RCV), index of individualities (IOI) and number of samples (N) with week-to-week intervals needed to estimate the underlying homeostatic set point (+/-15%) for BNP and NT-proBNP were calculated in pre-specified stable CHF patients. METHODS AND RESULTS: We measured plasma concentrations of BNP and NT-proBNP, clinical and laboratory variables in 20 CHF patients with a 7-days interval. Only patients considered to be in steady state were included. The CV(I) was 15% (BNP) and 8% (NT-proBNP). CV(T) was 16% (BNP) and 8% (NT-proBNP) and RCV was 43% (BNP) and 23% (NT-proBNP). IOI was 0.14 for BNP and 0.03 for NT-proBNP and N was 1 for BNP and 1 for NT-proBNP. CONCLUSIONS: Our data demonstrate that unexplained variation of BNP and NT-proBNP is low in CHF patients during steady state, which is a prerequisite for the use of these peptides for monitoring of the disease.