The terahertz region (1-10 THz) of the electromagnetic spectrum offers ample opportunities in spectroscopy, free space communications, remote sensing and medical imaging. Yet, the use of THz waves in all these fields has been limited by the absence of appropriate sources. Existing solid-state emitters, in fact, lack the general requisites of compactness, integrability and portability necessary for implementation in actual devices, and suffer from low output powers, limited tunability, and/or the necessity of liquid helium cryogenics. Research on THz semiconductor lasers has received new life in the last few years thanks to the development of quantum cascade (QC) lasers of ever increasing wavelength. Yet, fundamental problems related to the transition energy being below the optical phonon resonance and to efficient confinement of the emitted radiation have seriously hampered the realization of THz QC devices. With the help of detailed theoretical modelling and the adoption of a special partially metallic waveguide, we have now demonstrated a monolithic THz heterostructure laser based on interminiband transitions in the conduction band of a GaAs/AlGaAs QC heterostructure. Single mode emission has been achieved at 4.4 THz (/spl lambda/ /spl sim/ 67 /spl mu/m) with high output powers of more than 2 mW. Although operation is presently limited to 60 K, the low threshold current density of less than 300 A/cm/sup 2/ testifies the great development potential for higher temperatures.