A spin-current density-functional theory (SCDFT) is introduced, which takes into account currents of the spin-density and thus currents of the magnetization in addition to the electron density, the non-collinear spin-density, and the density current, which are considered in standard current spin-density-functional theory. An exact exchange Kohn-Sham formalism based on SCDFT is presented, which represents a general framework for the treatment of magnetic and spin properties. As illustration an oxygen atom in a magnetic field is treated with the new approach.