Limitations are discussed on the use in quantum dot (QD) lasers of the conventional method of determining internal loss coefficient alpha(int) and internal quantum efficiency eta(int) from a measured plot of the reciprocal slope efficiency versus the cavity length L. The limitations are imposed by the L-dependence of alpha(int) and eta(int) themselves. The effect of internal loss is quantitatively analyzed, which originates from the dependence of alpha(int) on the carrier density, with the latter being L-dependent. In short cavities, a plot of the reciprocal slope efficiency versus L can significantly deviate from a straight line, thus limiting the practicality of the standard procedure. For L longer than several hundred mu m, the limitations are strong in a single-QD-layer laser and moderate in a multiple-QD-layer laser. (c) 2006 American Institute of Physics.