The spontaneous emission rate from an In0.2Ga0.8As quantum well in a cylindrical dielectric cavity with submicron radii and without mirrors in the emission plane is calculated based on a rigorous description of the cavity modes and their interaction with the 2D electronic system. The rate is suppressed for radii much less than the peak optical wavelength (rcav≪&lgr;/n), enhanced by up to a factor of ≊8 forrcav≊&lgr;/n, and remains similar to its value in the absence of a cavity forrcav≫&lgr;/n. The emitted light produces a highly collimated vertical beam in the intermediate region with the majority of photons emitted into the guided cavity modes, and its spatial distribution broadens dramatically in the smallest structures. The global spectral width of spontaneous emission in cavities that can presently be fabricated is largely unaffected by the introduction of lateral dielectric boundaries. ©1995 American Institute of Physics.