The density fluctuations in poly(methyl methacrylate) are investigated by small-angle X-ray scattering (SAXS) as a function of temperature and microstructural state with time-resolved accuracy, by means of synchrotron radiation. We show for the first time the differences in electronic density fluctuations between quenched and annealed samples in relation to their dynamic behavior. At low temperature, the microstructural state slightly influences the electronic density fluctuations, whereas differences between both samples appear in the glass transition range. SAXS is thus essentially sensitive to fluctuations of dynamic nature. These features are understood and related to the differences observed directly in the relaxational behavior, as revealed by high resolution mechanical spectroscopy and differential scanning calorimetry measurements.