The crystal lattice was disordered when crystalline lactose (.alpha.-monohydrate) was dehydrated to the .alpha.-anhydrate by heating in air. The disorder parameter of the .alpha.-anhydrate obtained by desiccating .alpha.-monohydrate in methanol was smaller than that of the product obtained by heating it in air. This disorder was also induced by grinding the .alpha.-monohydrate, and the free energy level of the water of crystallization in the ground sample seemed to be higher than that in the intact sample. No structural change of amorphous lactose was observed at 30.degree. C in a P2O5 desiccator for 30 d [days]. During storage at 30.degree. C, 60% R.H. [relative humidity] for 24 h, crystal growth of .alpha.-monohydrate in the solid state occurred and the degree of the crystallinity reached .apprx. 75%. Further crystal growth hardly proceeded. Crystals of .beta.-anhydrate were also formed. The disorder parameter of this transformed lactose was larger than that of the intact sample. The degree of stress relaxation of lactose was reported to be small, but that of amorphous lactose was nearly equal to that of crystalline cellulose. The tablet hardness of amorphous lactose was .apprx. 10 times that of crystalline lactose. [Lactose has been widely used for solid pharmaceutical preparations and many grades are commercially available.].