Experimental start-up results of a 118 GHz TE(22,6) gyrotron are presented and compared with theory. The theoretical excitation regimes of competing modes are computed in the energy-velocity-pitch-angle plane. The start-up paths through the plane are determined by the time evolution of the beam parameters during the start-up phase and are seen to determine the cavity oscillating mode. Experimental results show that competition between the TE(22,6) and TE-(19,7) modes can be completely eliminated by using the proper start-up method in a case where a typical triode start-up results in oscillation in the competing TE-(19,7) mode. These new results, applicable to any arbitrary gyrotron cavity mode, are shown to be in excellent agreement with theory.