Optical losses in pillar microcavities have been measured directly by means of microreflectance spectroscopy. It was shown that optical losses of the cavities larger than about 3 &mgr;m were constant irrespective of the diameters. The lasing thresholds were also measured, and they were proportional to the cavity volume. This result is consistent with the measured constant optical losses. A drastic increase in optical losses has been observed when reducing the cavity diameters below 2 &mgr;m. The scattering losses due to sidewall roughness were considered mainly responsible for the optical losses. ©1996 American Institute of Physics.