M utations in ras and p53 are the most prevalent mutations found in human nonmelanoma skin cancers. Although some p53 mutations cause a loss of function, most result in expression of altered forms of p53, which may exhibit gain-of-function properties. Therefore, understanding the consequences of acquiring p53 gain-of-function versus loss-of-function mutations is critical for the generation of effective therapies for tumors harboring p53 mutations. Here we describe an inducible mouse model in which skin tumor formation is initiated by activation of an endogenous K-ras^G12D allele. Using this model we compared the consequences of activating the p53 gain-of-function mutation p53^R172H and of deleting the p53 gene. Activation of the p53^R172H allele resulted in increased skin tumor formation, accelerated tumor progression, and induction of metastasis compared with deletion of p53. Consistent with these observations, the p53^R172H tumors exhibited aneuploidy associated with centrosome amplification, which may underlie the mechanism by which p53^R172H exerts its oncogenic properties. These results clearly demonstrate that p53 gain-of-function mutations confer poorer prognosis than loss of p53 during skin carcinogenesis and have important implications for the future design of therapies for tumors that exhibit p53 gain-of-function mutations.