A relativistic calculation of the electron self-energy in a strong homogeneous magnetic field is presented, and the final expressions are calculated numerically for the Landau levels N =0,1,2,3 and both spin projections ( u = +I). For a given N > 0 the energy of the spin-up state (u = + 1) increases monotonically, but the spin-down ( u = -1) energy is negative for small fields, shows a minimum, and then increases for large fields; a crossing of levels appears for N > 0. The total decay rates for these states are also obtained, as well as the spin-flip transition probability for (N = 1, u = + l ) + ( N = 1, u = -1). It turns out that this transition rate is extremely small compared to the usual cyclotron emission rates.PACS number(s): 12.20.Ds, 03.65.Pm, 97.60.Jd