Selective inhibition of oncogenic targets and associated signaling pathways forms the basis of personalized cancer medicine. The clinical success of (V600E)BRAF inhibition in melanoma, coupled with the emergence of acquired resistance, underscores the importance of rigorously validating quantitative biomarkers of treatment response in this and similar settings. Because constitutive activation of BRAF leads to proliferation in tumors, we explored 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) PET to noninvasively quantify changes in tumor proliferation that are associated with pharmacologic inhibition of (V600E)BRAF downstream effectors and that precede changes in tumor volume.