Optical absorption by a dilute monodisperse ensemble of semiconducting nanoparticles with spherical shape embedded in a matrix is theoretically studied, taking into account the microscopic disorder, which produces a broadening and a redshift of the absorption spectra. We consider nanoparticles in the mesoscopic regime, where the diameter of the particles is much larger than the lattice constant, and macroscopic parameters characterizing the nanoparticles can still be defined. The macroscopic optical response of the medium is considered in the Maxwell-Garnett approximation, which is strictly applicable, if the particles form a cubic lattice. Disorder effects generated by deviations from this structure and resulting in an additional broadening and shift of the absorption spectra are neglected. It is shown that microscopic disorder generates a significant broadening and redshift of all resonances in the regime where the fluctuations of the transitions induced by disorder exceed the electron-electron correlation energy. Lowering the impurity concentration inside of the nanoparticles, yielding a weakening of the induced fluctuations, the electron-electron correlation energy can narrow effectively the resonance lines close to the band gap. In that regime the peak height of the lowest resonance is higher than the other ones. That spectral feature is not observed in systems containing semiconducting nanoparticles produced by wet-chemical methods and, consequently, we conclude that the measured broad spectral feature is not exclusively produced by the inhomogeneous size and shape distribution as usually assumed. Explicit calculations are performed for GaAs nanoparticles in the strong confinement or weak-coupling regime, respectively, in the framework of a two-band model and neglecting valence-band mixing effects. \textcopyright{} 1996 The American Physical Society.