A fluid model is used to derive the space-charge-limiting current of a relativistic electron beam drifting in a rippled-wall cylindrical waveguide. It is found that an enhancement of the space-charge-limiting current is found as long as optimum values of the ripple wavelength and amplitude are chosen. However, a very small ripple wavelength of the waveguide may reduce the space-charge-limiting current rapidly. The result should be of interest to the operation of backward-wave oscillators.