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The waveguide modes of metal-clad ridge-waveguide (MCRW) lasers are calculated rigorously by way of laterally coupled-mode equations. Thereby propagation constants of the waveguide modes are computed and compared to an approximate theory. Taking into account the complete set of local TE- and TM-eigenmodes, our analysis reveals that MCRW laser modes are mainly TE-polarized with a TM-component of some percent in case of narrow stripe lasers and it is shown that for strong lateral discontinuity of the waveguide geometry radiation losses appear owing to excitation of leaky TM-modes. Furthermore, modes losses caused by metal absorption are investigated and found to be in the order of 10 cm-1for highly conductive metallizations.