Thymocytes carrying MHC class I–restricted TCRs differentiate into CD8 T cells, while those recognizing MHC class II become CD4 T cells. The mechanisms underlying how MHC class recognition, coreceptor expression, and effector function are coordinated are not well understood. Since the tyrosine kinase Lck binds with more affinity to CD4 than CD8, it has been proposed as a candidate to mediate this process. By using transgenic mice with altered Lck activity, we show that thymocytes carrying a class II–restricted TCR develop into functional CD8 T cells when Lck activity is reduced. Conversely, thymocytes carrying a class I–restricted TCR develop into functional CD4 T cells when Lck activity is increased. These results directly show that quantitative differences in the Lck signal control the CD4/CD8 lineage decision.