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Background: Cancer antigen 15-3 is a protein that clinicians commonly measure to monitor outcomes and response to treatment in patients with breast cancer. However, cancer antigen 15-3 can also be elevated in other, benign and malignant conditions. Case presentation: A 73-year-old White woman with history of breast cancer presented to her primary care physician with right hip pain, and laboratory testing revealed elevated cancer antigen 15-3. Further workup with radiographic imaging revealed a large mass in her right kidney. The renal mass was subsequently removed, and the cancer antigen 15-3 level returned to normal. Conclusions: Elevation of cancer antigen 15-3 owing to causes other than breast cancer recurrence can be a potential diagnostic pitfall during a patient's follow-up. It is important for clinicians to be aware of the limitations of cancer markers and to utilize a combination of diagnostic tests for patient evaluation. [ABSTRACT FROM AUTHOR]