Drastic changes of fluorescence dynamics in pyrene-doped poly(methylmethacrylate) latex microspheres were observed with a time-resolved microspectroscopy system. Fluorescence decay curves of pyrene monomer emission from the microspheres consist of fast and slow decay components. Rate constants of the slow components are almost the same for all particles, which correspond to the value in free space. The fast components increase in the relative intensity and in the decay rate with decreasing particle size, which can be ascribed to cavity-enhanced spontaneous emission. The rate constant for a 2.4 μm sphere is increased by a factor of ∼17 compared to the bulk value. The results are discussed on the basis of the cavity quantum electrodynamic theory.