A simple theory is developed for the analysis of microstrip patch elements which are loaded at one or more points with lumped linear load impedances. The analysis is based on a cavity model in which the shape of the field distribution between the patch and ground plane is assumed to be well approximated by that of the resonant modes of a corresponding magnetic- and electric-walled cavity. The resonant mode of the loaded cavity is represented as an appropriate superposition of the modes of the corresponding unloaded cavity. The characteristic equation for the resonant frequencies of the loaded cavity is obtained in terms of the load impedances and the unloaded cavity multiport open-circuit parameters. An analysis of the input impedance of a rectangular microstrip element shorted at an arbitrary point has been implemented and the results show good agreement with experiment. An ancilliary result showing the equivalence between a thin strip and a circular cylinder model of a feed current distribution is also obtained.