Secondary membranous glomerulonephritis (MGN) is usually caused by drugs or systemic disorders that produce circulating immune complexes. However, some disorders that did not seem to form circulating immune complexes have been listed as underlying diseases of secondary MGN. The case of a 13-year-old boy with MGN and unilateral renal agenesis is presented. Renal histology showed segmental MGN with mesangial proliferation and mesangial electron-dense deposits, and no other underlying disorder except for unilateral renal agenesis. Patients with MGN, who have histological findings suggesting secondary MGN and no underlying disorder distinctly causing formation of circulating immune complexes, should not be defined as having idiopathic or secondary forms, but as having a ‘cryptogenic’ form.