Three monolithic circuits were designed and fabricated on GaAs substrates using CPW (coplanar waveguide) transmission lines. These circuits consist of two amplifiers and a frequency doubler and demonstrate the application of CPW in the Ka band. The active device used in these circuits is a 0.25- mu m-gate AlGaAs high-electron-mobility transistor defined by electron-beam lithography. Transmission line and substrate dimensions are chosen to avoid interaction with extraneous modes. The cascode configuration is shown to be a convenient gain element for use with CPW circuits where low-inductance RF grounding of the second gate is easily achieved. It is shown that CPWs are well enough characterized to be used in sensitive millimeter-wave circuits.