  We demonstrate laser frequency stabilization to excited state transitions
using cascade electromagnetically induced transparency (EIT). Using a room
temperature Rb vapor cell as a reference, we stabilize a first diode laser to
the D2 transition and a second laser to a transition from the intermediate
state to a Rydberg state with principal quantum number n=19 - 70. A combined
laser linewidth of 280 kHz over a 0.1 ms time period is achieved. This method
may be applied generally to any cascade system and allows laser stabilization
to an atomic reference in the absence of strong optical transitions.
