Our recent study of the performance of integrating spheres shows prominent UV induced fluorescence features that are associated with contamination of the diffusing wall material by hydrocarbons. Because of multiple reflections of the radiation inside the integrating sphere, fluorescence is induced multiple times with each reflection of the incident radiation by the wall. We report a simple theory on the fluorescence of integrating spheres by extending the well-known theory on integrating sphere throughput. The results indicate a strong dependence of fluorescence on the hemispherical reflectance of the diffusing material at both the excitation and fluorescence wavelengths as well as the geometry of the integrating sphere. Due to multiple reflection of the exciting radiation, a gain of more than an order of magnitude in fluorescence is possible compared with direct and single irradiation of a flat piece of the diffusing/fluorescing material.