Electromagnetic theory of open laser resonators is formulated in the domain of partially coherent optics. The theory is then used to find out the electromagnetic degree of coherence of the field in various situations. It is shown that if only one transverse mode is present in the steady-state condition, then the field is necessarily completely coherent in view of the recently introduced degree of coherence for electromagnetic fields [Opt. Express. 11, 1137 (2003)].