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Rationale:Renal complications in ankylosing spondylitis (AS) were rarely observed, and proteinuria associated with AS can be seen often due to amyloidosis in this kind of complications, while membranous nephropathy (MN) is seldom considered. This article reports a case of coexistence of AS and MN, to provide the exact relationship of these 2 entities and recognized some causes of renal involvement in AS.Patient concerns:A 44-year-old female presented with pain of the left leg for 4 years and pedal edema for 2 weeks.Diagnoses:AS was diagnosed according to the patient's clinical manifestation and sacroiliitis observed on computed tomography (CT) scan. Nephrotic syndrome was found and MN was diagnosed according to kidney biopsy in which thickened capillary loops were observed with light microscopy, granular deposits of IgG along the capillary wall were observed using immunofluorescence staining, and subepithelial electron-dense deposits were observed with electron microscopy. No other secondary causes of MN were found on extensive investigations.Intervention:Given the diagnoses, the patient received nonimmunosuppressive therapy for MN and adalimumab for AS.Outcomes:The patient got pain relief, as well as urinary protein reduction.Lessons:This case suggested a secondary MN in association with AS and the relationship between these 2 diseases needed more concern and further illumination.