Major Histocompatibility Complex class I molecules, MHC-I signal infection or transformation by engaging receptors on T lymphocytes. The spatial organization of MHC-I on the plasma membranes is important for this engagement. We and others have shown that MHC-I molecules, like other membrane proteins, are not uniformly distributed, but occur in patches in the plasma membrane. Here we describe the temporal details of MHC-I patch formation and combine them with the spatial details, which we have described earlier, to yield a comprehensive quantitative description of patch formation. MHC-I is delivered to the plasma membrane in Clathrin-coated vesicles, arriving at a rate of ∼2.5 X 10−3 μm−1 min−1 (or ∼2 arrivals per minute over the whole cell). The vesicles dock and fuse at non-random, apparently targeted, locations on the membrane and the newly-delivered MHC-I molecules form patches a few 100's of nm in diameter. The patches are maintained at steady state by a dynamic equilibrium between the rate of delivery and the rate of hindered (by components of the actin cytoskeleton) diffusion of MHC-I molecules out of the patches.