A 73-year-old male with nephrotic syndrome was admitted to our hospital. He was empirically treated with prednisolone, which resulted in the alleviation of proteinuria, hypoproteinemia, and pleural effusion. Thereafter, a computed tomographic scan revealed a mass lesion in the right-lower lung field. Finally, the patient died of multiple organ failure induced by disseminated intravascular coagulation. Adenocarcinoma of the lung and membranous nephropathy (MN) were revealed by autopsy. MN tends to occur in the elderly, and is also occasionally associated with solid tumors, such as lung and gastrointestinal cancer. Therefore, a malignancy survey may be useful in the management of cases with nephrotic syndrome in which MN is pathologically defined. However, the initiation of empirical treatment without a pathological diagnosis is not an exceptional phenomenon. Physicians should, therefore, bear in mind the potential association of malignancy and immediately and carefully investigate the potential presence of a malignancy in elderly patients with a new onset of nephrotic syndrome.