This paper discusses the effect of radiation losses in the responses of parallel-line mm-wave open microstrip filters. The analysis presented is based on a method that accounts for the presence of the substrate and the associated surface wave propagation. This analysis is very accurate for a wide frequency range, starting from a few GHz up to millimeter and submillimeter waves, where existing techniques fail to apply. The computer codes generated by this work can be effectively used in conjunction with available design packages to evaluate unwanted radiation losses and possibly eliminate them by slightly modifying the geometry of the filters. Comparison of theoretical results from the presented analytical method with experimental data shows excellent agreement.