Aims/hypothesis Pancreatic ductal adenocarcinoma (PDAC) can cause type 3C diabetes, known as PDAC-associated diabetes mellitus (PDAC-DM), but the mechanism is unknown. This study aimed to reveal the mechanism. Methods PDAC lesions from patients with or without PDAC-DM (n=4 in each group) were individually profiled for 23,512 mRNAs with microarrays. Bioinformatic analysis and in vivo and in vitro assays were then conducted. Results We determined that 2,778 genes were differentially expressed; over-representation of ten genes was validated with quantitative RT-PCR. The analysis of gene ontology showed that the differentially expressed secretory genes were related mainly to inflammation. High levels of a marker of inflammation (C-reactive protein [CRP]) and an inflammatory mediator (TNF super-family member 13 [TNFSF13]) were found in the serum of patients with PDAC-DM. After surgical resection of PDAC lesions, CRP and TNFSF13 levels significantly decreased (p<0.01). Furthermore, we found that the levels of TNFSF13 in PDAC lesions and TNFSF13 and CRP in serum were significantly correlated with the diabetic status of patients with PDAC-DM (p<0.01). Assays in vivo showed that after exposure to an inhibitor of inflammation (celecoxib), the fasting blood glucose level in the mouse model of PDACWenchao Gao, Yu Zhou and Qingyan Li contributed equally to this work Electronic supplementary material The online version of this article