We report on the design and fabrication of λ∼5.25 μm quantum-cascade lasers (QCLs) for very high temperature continuous-wave (cw) operation. Cw operation is reported up to a maximum temperature of 90 °C (363 K). Cw output power is reported in excess of 500 mW near room temperature with a low threshold current density of 1.4 kA/cm2 at 298 K. Room temperature average power of over 540 mW is reported at 50% duty cycle. A high thermal conductance (Gth) of 340 W/K cm2 is reported for cw QCLs. A finite element thermal model is used to investigate the Gth and maximum cw operating temperature of the QCLs.