Background: In patients with collagen disease, such as rheumatoid arthritis, steroids and immunomodulators are often used for treatment. These treatments increase the risk of adverse events, including infections; thus, cancer patients with collagen disease are often excluded from clinical trials of chemotherapy for cancer. Therefore, evidence is lacking regarding the safety and efficacy of chemotherapy for pancreatic cancer concomitant with collagen diseases. Methods: This was a multicenter, retrospective study in Japan. Patients who started first-line chemotherapy or chemoradiotherapy for unresectable pancreatic cancer between January 2015 and December 2019 and had concurrent collagen diseases were included. We evaluated adverse events (AEs), discontinuation of anticancer treatment due to AEs, progression-free survival (PFS) and overall survival (OS). Results: A total of 52 patients were enrolled from 20 institutions in Japan, and a total of 101 regimens were administered. Concomitant collagen diseases included rheumatoid arthritis (n=35), systemic lupus erythematosus (n=5), Sjögren's syndrome (n=4), vasculitis syndrome (n=3), dermatomyositis/polymyositis (n=2), polymyalgia rheumatica (n=1), systemic sclerosis (n=1), and Behçet’s disease (n=1). The most common regimens for pancreatic cancer were gemcitabine/nab-paclitaxel (GnP, n=40), S-1 monotherapy (n=18), gemcitabine monotherapy (n=15), modified FOLFIRINOX (n=10), S-1 concurrent radiotherapy (n=5), and FOLFOX (n=4). Treatment discontinuation due to AEs occurred in 21 regimens (20.8%). The most common causes of discontinuation were thrombosis in 4 regimens, interstitial lung disease (ILD) in 3 regimens, and fatigue in 3 regimens. Infection as an adverse event was observed in 7 regimens (6.9%). By regimen, GnP had the highest discontinuation rate for adverse events of 37.5% and an incidence of ILD of 15.0%. The median PFS was 5.6 months in first-line therapy, and the median OS was 11.1 months. By regimen, the median PFS (5.8 months) and median OS (13.3 months) for metastatic patients receiving first-line GnP were comparable to those in previous reports on GnP for patients without collagen disease. Conclusions: Patients with pancreatic cancer concomitant with collagen disease have a high discontinuation rate due to AEs, and attention should be given to the occurrence of thrombosis and ILD.