Introduction. The matrix of fiber‑reinforced composite (FRC), depending on the chemical composition, can absorb water. Water molecules diffusing into the spaces between the polymer chains can cause a change in the dimensions of the material. Such relaxation of the polymer network often leads to a change in composite parameters, including its mechanical strength. Aim. It therefore seems advisable to carry out tests on the water absorption of selected polymer materials used in dentistry for the construction of fixed and removable dental restorations. Material and Methods. The research used commercially available fiber‑reinforced composites: Podwiązka (ARKONA LFS, Poland), Dentapreg PFM (ADM, Czech Republic) and experimental material reinforced with ultra‑high molecular weight polyethylene (UHMWPE) manufactured in ARKONA LFS (Poland). Samples of the tested materials were stored in a solution of synthetic saliva and weighed on the 1st, 7th, 14th, 21st and 28th day of the experiment. Results. For the Podwiązka material, the results differed statistically significantly (p = 0.026) between the first and the second measurement of weight. For Dentapreg PFM, statistically significant results occurred between the 1st and 2nd measurements (p < 0.001), between 1st and 3ed (p < 0.001) and between 1st and 4th (p = 0.033). No differences were observed between the parameters measured for the UHMWPE material. Conclusions. Dentapreg PFM shows the highest absorption of water whereas Podwiązka demonstrates water absorption only between first and second measurements and an increase in its weight. UHMWPE the experimental material did not significantly increase its weight in any of the measurements carried out. The water sorption of all tested materials complies with the PN‑EN ISO 10477 standard.

Key words: water absorption, fiber‑reinforced composites (FRC), artificial saliva, weight change.