Aim: To assess the correlation between the amino-terminal pro-hormone brain natriuretic peptide (NT-proBNP) concentration in blood and urine during a period when actively adjusting the treatment of heart failure (HF). Methods: Plasma and urine analyses of NT-proBNP were compared in 51 patients on admission to and discharge from a nurse-led outpatient clinic where HF treatment was optimized. The median time between the two measurements was 42 days. Correlations were analyzed using linear regression, where R-2 is the degree of variability in the plasma NT-proBNP concentration that can be accounted for by the urinary NT-proBNP. Results: There was a statistically significant linear relationship between the urine and plasma concentrations of NT-proBNP on both occasions, but R-2 varied greatly depending on how the data were presented. The correlation between the raw data showed an R-2 of only 30%, and it almost doubled upon logarithm transformation, which shows that the variability (error) was concentration-dependent. Correction of the urinary NT-proBNP for urinary creatinine further increased R-2 for the logarithm-transformed correlation to 68% on admission and 76% on discharge. The highest R-2 (77%) was obtained when the relative changes in urinary NT-proBNP/creatinine between admission and discharge were compared with the corresponding relative changes in the plasma concentration. The sensitivity and specificity of the urine in indicating plasma concentration changes > 10% were 82% and 86%, respectively. Conclusion: Relative changes in plasma NT-proBNP could be reliably estimated from urine samples during a period of optimization of HF treatment.