AbstractIntroductionSystemic lupus erythematosus (SLE) complicated by multiple myeloma (MM) is a relatively rare clinical presentation, and is easily ignored due to their similar or even identical manifestation, which may lead to misdiagnosis and mistreatment.Case reportWe report two cases of SLE with MM. Case 1 was a 59‐year‐old male who was diagnosed with SLE 11 years ago. Abnormal kidney function was detected 4 months ago and a bone marrow aspirate revealed MM. He then received three cycles of bortezomib, dexamethasone, and chemotherapy with liposomal doxorubicin, and one cycle of lenalidomide plus dexamethasone. He died of infectious shock. Case 2 was a 58‐year‐old female who was diagnosed with SLE 27 years ago. After the onset of abnormal renal function 4 years ago, the patient was still treated according to SLE disease activity. When renal function rapidly deteriorated, serum and urine immunofixation electrophoresis was positive for IgG γ with free light chains and she was diagnosed with SLE complicated by MM. She did not agree to the treatment for MM as advised and was discharged from the hospital against medical advice. Case 2 died of cardiac failure. Thirteen cases of SLE with MM reported from 2000 to 2022 in PUBMED and Mendeley and our above two cases were reviewed. Among the 15 patients, 13 were females and 2 were males. The median age at the time of SLE with MM diagnosis was 50 years, and the median time to a delayed diagnosis was 7 years. The serum monoclonal immunoglobulin level was elevated and extramedullary manifestations of renal dysfunction were common.ConclusionsAn elevated monoclonal immunoglobulin level or newly unexplained renal dysfunction occurring in a patient with SLE should prompt monitoring and further screening of MM, rather than treatment as a secondary manifestation of SLE.