Objective: To evaluate water sorption and hygroscopic dimensional changes of five core buildup composite resins, and to provide references for material modification on water sorption. Methods: Five commercial core buildup materials (group A: Smart Dentin Replacement(TM); group B: NanoFil; group C: ParaCore(TM); group D: LuxaCore Z; group E: EmbraceCore(TM) Resin Cement) were fabricated to disk-shaped specimens: (15.0±0.1) mm diameter, (2.0±0.1) mm thickness (n=10). Specimens were thoroughly irradiated with curing lights. The initial mass in air was recorded, and the initial mass in deionized water was recorded. Five specimens of each group were immersed in deionized water for 28 d. They were weighed as a function of different immersion time (1, 2, 3, 4, 5, 6, 7, 14, 28 d). The mass in air was recorded, and the displayed mass in deionized water was recorded. Archimedes' principle was applied to calculate the dimensional changes. The other five specimens of each group were stored in artificial saliva and were tested by the same methods. Results: All specimens gained weight and hygroscopic changes during 28 d immersion. When stored in deionized water, the apparent mass change of group A [(10.6±0.9) μg/mm(3)] and the dimensional change of group A [(0.39±0.10)%] were the lowest, while the mass change of group E [(48.0±0.2) μg/mm(3)] and the dimensional change of group E [(3.16±0.13)%] were the highest (P<0.05). In artificial saliva, the lowest apparent mass change was found in group A [(11.8±1.0) μg/mm(3)] while the highest change was found in group E [(47.4±3.5) μg/mm(3)] (P<0.05). The lowest dimensional change was found in group C [(0.37±0.09)%] and the highest was found in group E [(3.07±0.19)%] in artificial saliva (P<0.05). Conclusions: Water sorption and dimensional changes vary in immersion fluids with different osmotic pressure. Water sorption and dimensional changes of core buildup composite resins are highly correlated with test materials in both of the deionized water and artificial saliva.