Background: The CA 19-9 tumor marker is commonly used alongside imaging to trend chemotherapy response in patients with pancreatic ductal adenocarcinoma. Presentation: We describe an unusual clinical case of metastatic pancreatic cancer who achieved a marked decline in CA 19-9 but paradoxically developed widespread pulmonary lymphangitic carcinomatosis leading to rapid clinical decline and death. Conclusions: This case highlights the limitations of using the CA 19-9 tumor marker in isolation.