Brain natriuretic peptide (BNP) is an useful marker for the differential diagnosis of dyspnea and pleural effusion of unknown origin. BNP elevation indicates myocardial overload. Two patients were admitted to our hospital with abnormal electrocardiogram or dyspnea. Left ventricular ejection fractions were below 40% and BNP levels were below 2.0 pg/ml in both patients. The diagnoses were dilated cardiomyopathy (DCM) based on coronary angiography, and pathological examination and suspected secretion insufficiency of BNP. Immunohistochemistry for BNP showed no significant difference in BNP expression in the myocardium between patients with DCM and elevated BNP and the present patients. We recommend that patients with intractable dyspnea undergo echocardiography at least once even if their BNP level is low.