In conventional surface‐emitting lasers, the photon density of states is essentially that of a bulk material and, therefore, spontaneous emission rates are the same as in bulk. However, in an ideal laser, one would like to have a single photon mode coincident with the peak in the material gain so that zero‐threshold lasing occurs. This possibility is examined for a surface‐emitting microcavity laser. The photon density of states for realistic GaAs‐based microcavity structures (4 μm length and 0.5–4 μm lateral width) is derived from solutions to the wave equation in the cavity. This density of states is used to study the properties of microcavity lasers with compressively strained quantum well active media by using the exact form of spontaneous emission rate in the rate equations. It is found that when the lateral dimension approaches 0.5 μm, all optical power is present in a single resonant photon mode (the lasing mode), although additional leaky and propagating modes exist. The spontaneous emission factors and threshold current densities are derived for a few microcavity lasers. It is found that for microcavities with lateral dimensions and aspect ratios practical in experimental realization, the ideal ‘‘thresholdless’’ behavior is not observed, although the threshold current density is reduced to ≊60 A/cm2 compared with 160 A/cm2 calculated for lasers with cross sections typical for conventional surface‐emitting lasers.