Objective — to determine the morphological features of IgG4-related lesions and to improve pathomorphological criteria for diagnosing various clinical variants of IgG4-related disease. Material and methods. Biopsy and surgical materials from 100 patients with tumor-like lesions at various sites (63 cases of IgG4-related lesion and 37 cases of non-IgG4-related inflammatory processes) were studied. Histological and immunohistochemical studies were conducted to determine the absolute counts of CD138+ cells, IgG+ and IgG4+ in the inflammatory infiltrates, as well as IgG4/IgG and IgG4/CD138 cell ratios. Results. When IgG4-related disease manifested, pancreatic, lacrimal, and salivary gland lesions prevailed. Brisk lymphoplasmacytic infiltration is characteristic for tissue damage in the eye, salivary glands, thyroid, pancreas, and skin. The formation of moiré fibrosis was specific to damages to the pancreas, liver and bile ducts, and eye tissues. Obliterative phlebitis is most often observed in pancreatic and salivary gland lesions. According to international criteria, the frequency of achieving the required level of IgG4+ plasma cells in each organ was high in lesions of the pancreas, bile ducts, and lymph nodes and that was low in lesions of the salivary and lacrimal glands and skin. The IgG4+/CD138+ and IgG4+/IgG+ cell ratios exceeded 40% in all cases. Conclusion. The morphologic diagnosis of IgG4-related disease is based on the detection of lymphoplasmacytic infiltration, moiré fibrosis, and obliterative phlebitis, as well as on the calculation of the absolute number of IgG4+ plasma cells in the inflammatory infiltrate and on the determination of IgG4+/IgG+ and IgG4+/CD138+ cell ratios. The number of IgG4+ plasma cells depends on the location of the lesion and on the phase of the process.