Induction of DNA damage by UVR is the key event in photocarcinogenesis. IL-12 and IL-23 are related heterodimeric cytokines consisting of a common p40 unit and a p35/IL-12 and a p19/IL-23 chain, respectively. Both exert immunomodulatory activities but are also found to reduce UVR-induced DNA damage presumably via induction of DNA repair. As both cytokines are also produced in the skin, they may mitigate the risk to develop UVR-induced skin cancer. This appears to be the case as mice lacking p40 were previously shown to be at higher risk for skin tumors upon chronic UVR exposure. As these mice express neither IL-12 nor IL-23, the individual effects of IL-12 or IL-23 could not be evaluated. Thus, mice lacking p35 (IL-12p35-/-) or p19 (IL-23p19-/-) were subjected to chronic UVR exposure. The Kaplan-Meier analysis indicated a significantly increased probability of tumor development in IL-23p19-/- but not in IL-12p35-/- mice. Taken together, in our model, loss of IL-23, but not of IL-12, enhances development of UVR-induced skin tumors, indicating that IL-23 but not IL-12 may counteract photocarcinogenesis. This may have impact on the development of future strategies utilizing antibodies against IL-12 and IL-23, respectively, for the treatment of inflammatory dermatoses.