The freezing and melting behavior of water in 15 types of poly (methyl methacrylate) (PMMA) hydrogel-hollow-fiber membranes utilized in artificial kidneys for ordinary hemodialysis, high flux hemodialysis, hemofiltration, and protein permeable hemofiltration, etc., was investigated using a differential scanning calorimeter, The freezing and melting temperatures of freezable water in the PMMA hydrogels were lower than those of bulk water, although the freezable water had almost the same mobility as that of bulk water, indicating that the freezable water was assigned to free water confined in the pores, i.e., freezable pore water, Pore size distribution curves regarding the PMMA membranes were then calculated from the melting endotherms and the freezing exotherms of the pore water via thermoporosimetry. The average pore radii of the membranes were also calculated from the pore size distribution curves, and they were found to be in good theoretical agreement with the average pore radii obtained from the water permeability, The number of pores was also found to correlate with the number obtained from the water permeability.