The effect of enzymatic cellulolysis on the moisture-adsorbing properties of cotton cellulose was investigated, using a highly purified endo-type cellulase from Streptomyces sp. KSM-26. The time course of changes in the hygroscopicity of cotton reflected the time course of changes in the extent of the amorphous state of cotton, which was estimated from the amount of water that could bind to cotton, as determined by differential scanning calorimetry, during the hydrolytic process. The endo-type cellulase increased the degree of freedom of cellulose chains in the amorphous regions of cotton at the initial stage of the hydrolytic reaction, leading to major improvements in the moisture-adsorbing properties of cotton. The cellulase then caused the rapid decrease in the size of the amorphous regions with the release of reducing sugars as endo-cleavages in the amorphous regions increased, which reduced the hygroscopicity. The decrease in the hygroscopicity of cotton during enzymatic hydrolysis was followed by a second increase, as the cotton became more amorphous as a consequence of further hydrolysis of the crystalline regions of cotton. The effect of cellulase on the apparent degree of polymerization and the physical strength of cotton became detectable at the later stage of cellulolysis.