Solid tumors involving glandular organs express mucin glycoprotein that is eventually shed into the circulation. As a result, these proteins can easily be measured in the serum and be used as potential tumor markers. The most commonly used tumor markers for breast cancer are CA27-29 and CA15-3, which both measure the glycoprotein product of the mucin-1 (MUC1) gene. CA27-29 has been approved by the US Food and Drug Administration for monitoring disease activity in breast cancer patients. Most oncology clinical practice guidelines do not recommend the use of tumor markers for routine surveillance of early stage disease but recognize their utility in the metastatic setting. We present a patient with stage IIIA breast cancer and preexisting hypersensitivity pneumonitis who was found to have an elevated serum tumor marker CA27-29. After successful curative intent treatment of her early stage breast cancer, she developed gradual and progressive worsening of her lung disease with eventual development of severe pulmonary fibrosis requiring bilateral lung transplantation. As part of the pretransplant evaluation, she was found to have an elevation of serum tumor marker CA27-29. While the diagnostic evaluation, including imaging studies, was negative for the presence of recurrent disease, the serial serum tumor marker CA27-29 levels remained persistently elevated. The decision was made for her to undergo bilateral lung transplantation. Shortly after surgery, her CA27-29 tumor marker level returned to normal range, and it has continued to remain in the normal range with no evidence of breast cancer recurrence.