Assuming that the differential gain is a linear function of temperature, a formula is derived to describe the threshold current as a function of temperature. A maximum operating temperature, T-max, which is related to the fundamental physical properties (such as differential gain, free carrier loss and intervalence band absorption) of the lasers appears naturally in the formula, at which lasing ceases. To experimentally investigate the relationship, studies were carried out on 1.3-mu m strained-layer multiple quantum-well (SL-MQW) lasers with variant 0.7% compressively strained wells. The formula shows a good correlation with threshold current versus temperature data over the temperature range 200 K-50 K.