CA-19.9 (Carbohydrate Antigen 19.9) is a tumor marker commonly associated with pancreatic cancer. It is widely used to monitor disease progression, evaluate response to therapy, and detect recurrence. While elevated levels are strongly linked to pancreatic cancer, they may also be present in other gastrointestinal malignancies such as bile duct, stomach, and colorectal cancers. Non-cancerous conditions, including liver cirrhosis, gallstones, and pancreatitis, can also cause an increase in CA-19.9, making it a useful but not entirely specific marker. When combined with imaging and other diagnostic methods, CA-19.9 helps guide treatment decisions.