Brain natriuretic peptide (BNP) level is elevated in heart failure and reflects its severity. It is unknown why some patients have extremely high BNP levels. The authors retrospectively reviewed data on 179 consecutive patients whose BNP levels fell within one of several predetermined ranges: mild elevation, 500 to 1000 pg/mL (n=82); moderate elevation, 2000 to 3000 pg/mL (n=48); and high elevation, 4000 to 20,000 pg/mL (n=49). The statistical analysis was conducted with the unpaired t test and Pearson's correlation coefficient. Adjustments were made for age, sex, and serum creatinine level. Patients with moderate BNP elevation were more symptomatic and had more advanced structural and hemodynamic changes than did patients with lower BNP elevation. Characteristics of the high BNP level group did not differ from those of the moderate BNP level group. Serum creatinine level correlated with BNP level, but neither age nor sex did. High BNP level (4000–20,000 pg/mL) is determined more by renal dysfunction than by the severity of heart failure.