Abbreviations & Acronyms CT = computed tomography ESRD = end-stage renal disease ICI = immune checkpoint inhibitor irAE = immune-related adverse event mRCC = metastatic renal cell carcinoma RCC = renal cell carcinoma

Introduction Methotrexate has been reported to increase the risk of lymphoproliferative disorders. We report a rare case who was clinically diagnosed with methotrexate‐associated lymphoproliferative disorders of the kidney. Case presentation A 77‐year‐old patient with rheumatoid arthritis had taken low‐dose methotrexate for 13 years. The patient developed left renal mass 3 cm in size and multiple pulmonary nodules. Initially, renal malignant tumor with lung metastases was considered and the renal biopsy was planned. However, under possible diagnosis of methotrexate‐related lymphoproliferative disorder, we withdrew methotrexate treatment at first and then observed spontaneous regression of the tumorous lesions of the kidney and lungs. Conclusion Although methotrexate‐related lymphoproliferative disorder in kidneys is very rare, our case advocates the importance of a relevant differential diagnosis of methotrexate‐related lymphoproliferative disorder under the setting of long‐term treatment of methotrexate for rheumatoid arthritis.