The optical properties of buried‐heterostructure Ga1−xAlxAs lasers are carefully studied during accelerated aging at 70 °C. It is shown that neither kinks nor self‐pulsation is observed in the majority of the lasers after long‐term aging, in contrast to the observations in conventional stripe‐geometry lasers. The observed stable operation of buried‐heterostructure lasers may be attributed to the transverse‐mode stability and small cavity size intrinsic to the lasers.