The ability to noninvasively quantitate tumor burden from conditional (Cre/loxP-dependent) mouse cancer models would greatly increase their range of useful applications. We now report the generation of a reporter mouse that enables visualization of spontaneous tumor development from pre-existing conditional mouse tumor models via in vivo bioluminescence imaging. We demonstrate that bioluminescence can be "switched-on" in a Cre-dependent manner in every organ analyzed, and that this gives rise to between a 4 and 6-log increase in light emission per mg of wet tissue weight. Furthermore, we highlight the utility of this reporter by showing that it can be used as a sensitive means to measure spontaneous Kras2(v12)-induced lung tumorigenesis in a pre-existing mouse model of non-small cell lung cancer. Taken together, our results suggest that this reporter may be combined with a wide-range of other Cre/loxP tumor mouse models, irrespective of their tissue specificity and render them immediately amenable to longitudinal monitoring of tumor growth and therapeutic response with a noninvasive in vivo imaging approach.