Photochemical hole-burning phenomenon is discussed with regard to its application in high-density optical memory systems. A theory is introduced to express the recording density limit of this memory as a function of readout time and material parameters. Experiments in dye-doped polymer systems show that the main factor limiting the attainable recording density is hole filling brought about by the irradiation of another wavelength. This hole filling occurs because the doped chromophore in an amorphous matrix has various energy levels. Extent of hole filling is related to the molecular structure of the chromophore. When a chromophore is rigid and does not have a low energy vibration mode, the extent of filling becomes smaller.