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Introduction: Multiple myeloma (MM) is an incurable bone marrow (BM)-based cancer involving clonal plasma cells. Most patients show elevated levels of serum monoclonal protein (sMP) and kappa or lambda serum free light chains (sFLCs) at diagnosis. However, around 1–2% of patients, termed nonsecretory, do not produce these biomarkers. As the disease progresses, more patients may become unevaluable using conventional markers, requiring invasive and expensive procedures like BM biopsies and positron emission tomography-computed tomography (PET-CT) scans for assessment and highlighting the need for alternative methods to monitor disease progression. Case Presentation: We present a case report of an MM patient who developed nonsecretory disease during his second line of treatment when he complained of new rib pain; progressive disease was then confirmed on a PET-CT scan. The patient showed an increase in his serum B-cell maturation antigen (sBCMA) levels whereas his conventional myeloma markers did not detect disease activity (sMP remained undetectable and involved sFLC level was normal). After starting a new treatment regimen, his rib pain disappeared, PET-CT scan improved, and sBCMA levels decreased. Upon relapse, he developed increased rib pain with a rising sBCMA level; his conventional myeloma markers did not detect disease activity. After changing to a new regimen, his rib pain improved, and this was accompanied by a decrease in his sBCMA levels. Conclusion: Thus, this case exemplifies the potential for sBCMA to provide a non-invasive method for monitoring MM patients who develop nonsecretory disease.