NPe6 is a novel second-generation photosensitizer used for photodynamic therapy (PDT). PDT using NPe6 and diode laser (664 nm) induces cell death, inflammatory reactions, immunological responses and damage to the microvasculature. In this study, we evaluated the influence of the immunological responses and of enhanced angiogenesis on the anti-tumor effect of NPe6-PDT using cytokine-overexpressing Lewis lung carcinoma (LLC), LLC-IL-2 cells both in vitro and in vivo. We showed by DNA microarray analysis in vitro that IL-2 and GADD-45alpha (growth arrest and DNA damage 45 alpha) mRNA expressions were induced by 3 h after NPe6-PDT applied at a dose killing 90% of the cells (LD90). IL-2-overexpressing cells (LLC/IL-2 cells) were resistant to the loss of clonogenicity as compared to the parental LLC cells in vitro. Furthermore, in female C57BL/6 mice, NPe6-PDT produced a cure rate of 66.7% in LLC tumors, whereas the cure rate was only 16.6% in LLC/IL-2 tumors, and overexpression of IL-2 caused failure of NPe6-PDT, with tumor recurrence, in vivo. These results suggest that IL-2 expression may play an unfavorable role in attenuation of the antitumor effect of NPe6-PDT. It has been reported that the expression of vascular endothelial growth factor (VEGF), in particular, may cause tumor recurrence after PDT and exert unfavorable effect in relation to attenuate the anti-tumor activity of PDT. Results of immunohistochemical analysis of LLC/IL-2 tumors have revealed that the expressions of GADD-45alpha and VEGF are induced in these tumors after PDT, and in particular, 12 h after PDT, the expression levels were much higher as compared with those in the LLC tumors. The results of our studies using in vitro and in vivo models suggest that the cell death caused by PDT was inhibited by induction of GADD-45alpha expression and that tumor recurrence was promoted by the enhancement of VEGF expression mediated by IL-2 upregulation. Therefore, it is speculated that the use of an IL-2 inhibitor may improve the efficacy of NPe6-PDT.