The threshold current and quantum efficiency are calculated for a far‐infrared injection laser, based on intersubband transitions in GaAs single quantum wells. By considering the balance of gain against the free carrier and transverse optical (TO) phonon absorption, and the intersubband transition time in the intermediate to the far infrared, the wavelength dependence of the threshold current is calculated. We find that for the wavelength range 50–120 &mgr;m, the required threshold currents are the lowest and have reasonable values of 103–104A/cm2. The threshold quantum efficiency in that range is expected to be ∼10−4.