The theory and experiment of a millimetre wave single-cavity abreast two-device power combiner are studied. The external circuit impedance looking outside from the two active devices, which are abreast-mounted in the rectangular waveguide cavity, are analysed and computed for the first time. Numerical results are given, which are very useful for the design and development of millimetre wave power combiners. The experimental results for a millimetre wave single-cavity abreast two Gunn devices power combiner at about 36 GHz are also given.