Relative vertical displacements and average relative vertical velocities have been determined for bench marks located throughout Yellowstone National Park (YNP) by comparing geodetic leveling measurements made in 1923 (second order), 1936 (third order), 1941 (third order), 1955 (third order), 1960 (first order), and 1975--1977 (first order). The 1923--1975 data indicate uplift greater than 400 mm (7.7 mm/yr) of an area approximately 15--20 km wide and at least 40 km long which is centered along the northeast trending axis of the 600,000-year-odd Yellowstone caldera; the maximum 1923--1975 vertical displacement in this area is 726 mm (14.0 mm/yr). The existence of a well-defined and untilted Yellowstone Lake terrace implies that the 1923--1975 average vertical velocities cannot have been maintained for more than a few hundred years. The 1955--1977 data from central YNP indicate that the northeastern caldera half has risen relative to the southwestern half at an average velocity of about 8 mm/yr; a similar bias is present in the 1923--1975 data but it is less pronounced. The 1923--1975 data near Norris Junction, may be interpreted in terms of deformation associated with the 1959 Hebgen Lake earthquake and the 1975 Yellowstone Park earthquake. A composite profile of the 1936--1977 and 1941--1977 average vertical velocity data which skirts the edge of the Yellowstone caldera in northeastern YNP is most easily explained as the result of random leveling error and/or bench mark instabilities. The apparent youth of the 1923--1975 Yellowstone uplift and its close association with the Yellowstone caldera suggest that the most likely cause of the uplift is a recent (not more than 500 years ago) influx of molten material into the upper crust beneath the caldera.