In a gyrotron, it is difficult to accurately predict in advance where the resonant interaction between the electrons and outgoing radiation stops. For accurately calculating the start currents for the interacting modes, the exit coordinate has to be fixed where the resonant interaction stops. This paper discusses the difficulty in fixing the exit coordinates for studying start currents in an overmoded gyrotron. Start currents are studied for the operating and the most dangerous parasitic mode of a 140 GHz gyrotron being developed by Communication and Power Industries (CPI). Calculations show that the start currents vary over considerably large values with varying exit coordinates that can cause difficulties in predicting which mode dominates the mode competition. © 2003 American Institute of Physics