The electroluminescent spectra and the light intensities of Alq3-based organic light-emitting diodes were measured as a function of the thickness of the light-emitting layer. A maximum in the light intensity occurs when the thickness of this layer is approximately 600 Å, whereas a minimum in the light intensity occurs at about 1400 Å. The electroluminescent spectra were also found to vary strongly with the thickness of the light-emitting layer. Computer simulation, based on wide-angle interference of light-emitting dipoles from a reflective mirror, can account for the experimental observations.