The temperature dependence of threshold current in graded‐index, separate‐confinement‐heterostructure, single quantum well lasers has been investigated and analyzed. The conventional parameter used to describe this temperature dependence,T0, is measured and shown to increase with cavity length. The temperature dependences of the loss coefficient &agr; and the differential gain &bgr; have also been measured. Both parameters decrease linearly with temperature in the range 20–70 °C. Competition between &agr;(T) and &bgr;(T) is shown to account for the dependence ofT0on cavity length and to suggest guidelines for designing highT0lasers.