The cavity length dependence of the external differential quantum efficiency and the threshold current density was investigated for ZnCdSe/ZnSe single-quantum-well separate-confinement-heterostructure laser diodes at 77 K. An internal loss of 1.5 cm-1, and internal quantum efficiency of 61% for stimulated emission, a transparency current density of 1.8 kA/(cm2 μm) and a gain factor of 0.36 cm2 μm/A were obtained. The transparency current density is 3-4 times larger than that for bulk GaAs. Improvement in the internal quantum efficiency for spontaneous emission will lead to reduction of the operation current.