The cw operation of chirped-superlattice quantum-cascade lasers emitting at λ∼67 μm (4.4 THz) is analyzed. Collected (min. 33% efficiency) output powers of 4 mW per facet are measured at liquid helium temperatures and a maximum operating temperature of 48 K is reached. Under pulsed excitation at duty cycles of 0.5%–1%, slightly higher (10%) peak powers are reached, and the device can be operated up to 67 K. Low threshold current densities of 165 and 185 A cm-2 are observed in pulsed and cw operation, respectively. The operation of the laser is examined using the Hakki–Paoli technique to estimate the net gain of the structure. © 2003 American Institute of Physics.