Background <p>Precise staging is essential for treatment options and patient benefits in individuals with unresectable pancreatic ductal adenocarcinoma (PDAC). Laparoscopic biopsy is a minimally invasive procedure used to identify radiographically occult metastases and accurately stage unresectable PDAC. The cancer antigen 125 (CA125) levels change rule after laparoscopic biopsy and its correlation with overall survival (OS) in patients with unresectable PDAC remains unclear.</p> Methods <p>We analyzed electronic medical records and follow-up data of patients diagnosed with unresectable PDAC by laparoscopic biopsy. Dynamic measurement of CA125 levels was used to analyze the CA125 levels change rule after laparoscopic biopsy and its prognostic significance.</p> Results <p>A total of 123 patients were ultimately included in our study, categorized into the unresectable locally advanced (UR-LA) and unresectable metastasis (UR-M) groups. In the UR-M group, CA125 levels were significantly elevated at 2 weeks after the first chemotherapy compared to preoperative levels. These elevated levels did not correlate with poor OS. Although CA125 levels decreased significantly at 4 weeks after the first chemotherapy, the values ≥ 35 U/mL were correlated with poor OS. A similar CA125 levels change rule was observed in the UR-LA group, but CA125 levels had no prognostic value at each time point. In patients with UR-M PDAC, a drop in CA125 levels below 35 U/mL within 4 weeks after the first chemotherapy was associated with better OS, regardless of whether the values exceeded 35 U/mL in the early postoperative period.</p> Conclusions <p>The CA125 levels change rule after laparoscopic biopsy can be used to predict the prognosis of patients with unresectable metastatic PDAC. CA125 levels remaining above 35 U/ml for longer than 4 weeks after the first chemotherapy is associated with a poor prognosis.</p>