We present a catalog of 345,000 stars with proper motions \mu \geq 20 { { { mas } } { { yr } } ^ { -1 } } and magnitudes r ^ { \prime } \leq 20 , drawn from the intersection of the SDSS Data Release One and USNO-B .
We first use SDSS quasars to correct the proper motions in each of the two source catalogs , finding that these corrections for systematics are comparable to the statistical errors .
The combined proper motions have errors of \sigma _ { \mu } = 4.0 { { { mas } } { { yr } } ^ { -1 } } , significantly better than any previous catalog at these magnitudes .
While cross-correlating SDSS and USNO-B removes the vast majority of the very large number of spurious proper-motion stars in each , some contamination remains , especially at faint magnitudes , r ^ { \prime } \gtrsim 19.5 , and high proper motions .
We present a diagrammatic method to estimate the level of contamination in selected subsets of the catalog , which should be useful for choosing appropriate selection criteria for various applications .