This study is concerned with a new type of dominant mode power leakage from uniform lengths of printed-circuit waveguides, such as microstrip line, slot line, and coplanar waveguide. These leakage effects, which are presently not recognized and can therefore be unexpected as well as undesired, occur at higher frequencies, and are therefore important for millimeter-wave integrated circuits and high-speed circuits. The cross talk and coupling that result can ruin the performance of a complex, high-density circuit unless the leakage effects are understood and controlled. The leakage effects reported here occur on uniform lengths of printed-circuit waveguide on which the dominant mode is purely bound at ordinary microwave frequencies but becomes leaky at higher frequencies; they appear in addition to the leakage produced by conversion into surface waves at various discontinuities. The nature of the leakage is described, and numerical results are presented for several structures as illustrations of the variety of ways this type of leakage can arise.