The effect of electrostatic adiabatic electron trapping in the presence of a depressed collector has been included in gyrotron efficiency computations. The results are presented as general contour plots in the control parameter plane. A condition for trapping is derived in terms of the retarding voltage. The overall electron efficiency as a function of the retarding voltage has been calculated for a representative set of control parameters including the effect of electron velocity spread. It is found that the onset of trapping seriously decreases the efficiency.