Lasing threshold modes are studied experimentally for microcavity lasers fabricated to operate either as planar cavities, in which the mode is established by the emitter distribution, or as index‐guided modes in the planar cavity with weak three‐dimensional confinement. For no lateral index guiding, the lasing mode area is suggested to depend on the gain saturation. For lateral index confinement to a small area, spectral and radiation pattern measurements show that the spontaneous emission coupling is increased, and reduces the lasing threshold even in the presence of an increased loss. ©1995 American Institute of Physics.