Pancreatic cancer is characterized by its desmoplastic tumor microenvironment and the infiltration of immunosuppressive cells. It is a devastating disease where most patients are diagnosed at a late stage and the treatment options are few. The development of new treatments is surly needed. One treatment option explored is the use of immunotherapy with the intent to activate the immune system and change the balance from pro-tumor to anti-tumor. This thesis presents the idea of using oncolytic adenoviruses called LOAd-viruses that are armed with immunostimulatory- and microenvironment-modulating transgenes. For effective treatment of pancreatic cancer, the virus needs to be able to be given in addition to standard therapy, the chemotherapy gemcitabine. In paper I, the immunomodulatory effect of gemcitabine was evaluated in blood from pancreatic cancer patients receiving their first 28-day cycle of treatment with infusions day 1, 8 and 15 followed by a resting period. Gemcitabine reduced the level of immunosup-pressive cells and molecules but the effect did not last throughout the resting period. On the other hand, gemcitabine did not affect the level or proliferative function of effector T cells indicating that gemcitabine could be combined with immunotherapy. The LOAd700 virus expresses a novel membrane-bound trimerized form of CD40L (TMZ-CD40L). In paper II, LOAd700 showed to be oncolytic in pancreatic cancer cell lines as well as being immunostimulatory as shown by its capacity to activate dendritic cells (DCs), myeloid cells, endothelium, and to promote expansion of antigen-specific T cells. In paper III, LOAd703 armed with both 4-1BBL and TMZ-CD40L was evaluated. LOAd703 gave a more profound effect than LOAd700 on activation of DCs and the virus was also capable of reducing factors in stellate cells connected to the desmo-plastic and immunosuppressive microenvironment. In paper IV, LOAd713 armed with TMZ-CD40L in combination with a single-chain variable fragment against IL-6R was evaluated. The virus could kill pancreatic cancer cells lines through oncolysis and could also reduce factors involved in desmoplasia in stellate cells. Most interestingly, LOAd713 could reduce the up-regulation of PD-1/PD-L1 in DCs after CD40 activation. Taken together, LOAd703 and LOAd713 seem to have interesting features with their combination of immunostimulation and microenvironment modulation. At present, LOAd703 is evaluated in a clinical trial for pancreatic cancer (NCT02705196).