Purpose: The properties of polymer composites depend on their structures. Good initial properties are often not enough because they change under environmental conditions. Thus, studies of functional properties should provide information about their initial properties and behaviour under operating conditions. The study aimed to determine the effect of the structure of dental composites and the environmental hydrothermal conditions on their mechanical strength. Design/methodology/approach: Light-cured polymer matrix ceramic composites (LC PMCCs) were investigated. Commercially available composites and experimental materials with different filler particle contents were tested. Compressive strength, three-point and biaxial flexural strength tests were carried out. The tests were performed using composites without a load history, exposed to a moist environment and hydrothermal ageing. Findings:In most cases, changes in compressive strength under the effect of the moist environment and hydrothermal ageing were non-significant. Compressive and three-point flexural strength values obtained for universal-type materials were higher than those obtained for flow-type composites. In contrast, higher values of biaxial flexural strength characterised the latter. Hydrothermal ageing caused the greatest decrease (approx. 60%) in the three-point flexural strength of universal-type composites. The strength degradation of flow-type materials was about 40%. Research limitations/implications: It is unknown whether the effects of fatigue due to mechanical and hydro-thermal loads are additive, i.e., whether the principle of superposition applies in the case of combined action of these loads or whether a synergy phenomenon occurs. In the next stages, experiments can be carried out involving the simultaneous operation of both types of force. Practical implications: The issue of durability is well established in the field of machine operation and, to a lesser extent, in relation to medical devices. Understanding the importance of the durability of medical facilities is not common. This interdisciplinary project may contribute to the dissemination of knowledge in this field. Originality/value: Based on the obtained test results, it should be concluded that the based on the change in the elastic modulus.