Mesenchymal stem cells (MSCs) are multipotent cells that can be obtained from different tissues, including bone marrow, adipose tissue, umbilical blood, Wharton's jelly, and dental pulp. Due to their differentiation potential, regenerative and immunosuppressive properties, as well as ability to expand under in vitro conditions, these cells represent a promising therapeutic tool for regenerative medicine. However, the basic prerequisite for the therapeutic utilization of MSCs is obtaining a sufficient amount. While this may be achieved by prolonged cultivation, long-term culture of MSCs is associated with accumulation of morphological and functional changes. In our study, we focused on analyzing morphological and biological changes of cultured adipose tissue-derived stem cells over 30 passages. We performed morphological analysis using light and electron microscopy, as well as analysis of selected biological properties (expression of surface antigens and selected genes involved in cell regulation and apoptosis, cell cycle, and cell senescence) every 5 passages. Our results showed that long-term expansion leads to significant changes in morphology and affects proliferation kinetics and the cell cycle. On the other hand, the MSCs maintained a prototypical immunophenotype, normal cell cycle and apoptosis regulator function, and maintained a low level of telomerase activity during later passages.