The intensity distribution of the extended diffuse regions observed in the x-ray diffraction pattern of anthracene single crystals was compared with intensity distributions calculated on the assumption of independent rigid-body vibrations of the anthracene molecules. The calculations included both translational and rotational vibrations of the molecules, as well as the effect of vibrational anisotropy on the diffuse pattern. In spite of the satisfactory overall agreement between observed and calculated distributions, important discrepancies were found which could not be explained on the assumption of independent molecular vibrations. The Faxén-Waller theory, however, gives adequate explanation of all observed effects and also accounts for the limited success of the theory of independent vibrations. This apparent success is the result of an integration over large parts of the Brillouin zones when recording the diffuse pattern. Various crystallographic and physical applications of the extended diffuse regions are discussed and inherent limitations are pointed out.