The Hypervelocity Star survey presents the currently largest sample of radial velocity measurements of halo stars out to 80 kpc . We apply spherical Jeans modeling to these data in order to derive the mass profile of the Galaxy . We restrict the analysis to distances larger than 25 kpc from the Galactic center , where the density profile of halo stars is well approximated by a single power law with logarithmic slope between -3.5 and -4.5 . With this restriction , we also avoid the complication of modeling a flattened Galactic disk . In the range 25 < r < 80 kpc , the radial velocity dispersion declines remarkably little ; a robust measure of its logarithmic slope is between -0.05 and -0.1 . The circular velocity profile also declines remarkably little with radius . The allowed range of V _ { c } ( 80 \mathrm { kpc } ) lies between 175 and 231 \mathrm { km s } ^ { -1 } , with the most likely value 193 \mathrm { km s } ^ { -1 } . Compared with the value at the solar location , the Galactic circular velocity declines by less than 20 % over an order of magnitude in radius . Such a flat profile requires a massive and extended dark matter halo . The mass enclosed within 80 kpc is 6.9 ^ { +3.0 } _ { -1.2 } \times 10 ^ { 11 } \mathrm { M } _ { \odot } . Our sample of radial velocities is large enough that the biggest uncertainty in the mass is not statistical but systematic , dominated by the density slope and anisotropy of the tracer population . Further progress requires modeling observed datasets within realistic simulations of galaxy formation .