Stroke volume is mainly determined by preload, afterload and contractility. Accordingly, measuring cardiac preload provides essential information for treatment of hemodynamically unstable patients. Hemodynamic monitoring is widely used to measure cardiac preload, but the monitoring method is time-consuming and invasive. It is therefore important to establish a simple and non-invasive test for monitoring cardiac preload. Brain natriuretic peptide (BNP), which lowers systemic vascular resistance, is synthesized as proBNP in response to myocardial wall stretch, and blood BNP has been used as an indicator of preload. Here, we measured blood level of N-terminal proBNP (NT-proBNP), which is generated during processing of proBNP, because NT-proBNP is stable and easily measured at the bedside. To assess the correlation between blood NT-proBNP and preload, we also measured the global end-diastolic volume index (GEDVI) that reflects the cardiac preload. GEDVI was calculated with the volumes in all chambers of the heart at the time of end-diastole. Eight male patients (57.6 ± 25.3 years old) with high volume load (1,000 ml within 4 hours) were included in the present study: 3 subjects with burn, 3 subjects with sepsis, a patient with alcoholic ketoacidosis and a resuscitated patient. Blood levels of NT-proBNP were 1,316.3 ± 1,154.5 pg/ml (47 blood samples from the eight patients; the normal range, < 125 pg/ml). Notably, the increase in the NT-proBNP levels was associated with the increased GEDVI (r = 0.61, p < 0.0001). Therefore, blood NT-proBNP may be a good indicator of cardiac preload in patients with high volume load.