<p>A histological study of structural changes in the thymus of female Wistar rats with chemically induced breast cancer (BC) after photodynamic therapy (PDT) and a combination of PDT and surgical treatment was conducted. After PDT, the thymus showed restoration of the cortical and medullary substance areas, as well as glandular tissue, to the values of the intact control in comparison with the pathological control (BC without treatment). PDT influences both positive and negative selection processes and the activity of T-lymphocyte differentiation processes (the number of epithelial cells and macrophages increases, the number of cells with pyknotic nuclei in the cortical and medullary substance decreases). In the cortico-medullary zone of the thymus, the number of small and medium lymphocytes increases, which may indicate an increase in lymphocyte migration from the thymus. After PDT and subsequent resection of a BC, structural changes in the thymus may indicate a decrease in its lymphopoietic function, activity of both positive and negative T-cell selection processes, as well as a decrease in the activity of T-lymphocyte differentiation processes and their migration from the thymus.</p>