Objectives: Carbohydrate antigen 19-9 (CA19-9) is a sensitive and specific serum marker in pancreatic cancer. Our retrospective analysis aims to evaluate CA19-9 decrease in patients with metastatic pancreatic cancer treated in ACCORD11/PRODIGE4 (FOLFIRINOX vs. gemcitabine). Methods: A total of 342 patients were treated. CA19-9 was measured at 8 weeks (±2) in 160 patients from a total of 282 with abnormal CA19-9 values at baseline (gemcitabine arm, n = 75; FOLFIRINOX arm, n = 85). In the present study, 8-week CA19-9 decrease or greater CA19-9 decrease according to the 20 and 90% thresholds were analyzed. Progression-free survival (PFS) and overall survival (OS) were estimated in each subgroup. Results: In the FOLFIRINOX arm, patients with an 8-week CA19-9 decrease or greater CA19-9 decrease ≥20% showed improved median OS, PFS, and objective response rate. In the overall study population, median OS and PFS were significantly improved in patients with an 8-week CA19-9 decrease ≥20% (vs. <20%). The 8-week CA19-9 decrease was predictive of PFS (interaction test significant according to treatment arm; p = 0.006). Conclusion: An 8-week CA19-9 decrease ≥20% is a prognostic factor for OS and PFS. The 8-week CA19-9 decrease (20% threshold) is predictive of PFS. It could help to evaluate the efficacy of FOLFIRINOX and gemcitabine regimens.