The article provides grounds for a new, expanded definition of a concept «population hematology». It is not only a hematological subsection of the traditional population and epidemiological medicine. It is focused on patients populations as its primary object; however, its uniqueness is in the fact that its primary object is the nested population hierarchy, such as blood cells, cell clones and human groups. The heterogeneity of the samples and changes in the heterogeneity with the course of time are most important. Taking into account the time factor, staging, and long-term observation are unique characteristics of the population hematology methodology. Mechanism of population formation, leaving, and changes in its composition significantly affect the analysis of clinical and population study findings. The peculiarity of this field of medical knowledge is interaction, overlapping of target populations. Donor-recipient, patient-doctor, blood cells-body, virus-carrier-host, blood component-sample, donor-recipient sample, etc. - these are only some of alive and lifeless study objects which may undergo a thorough study. Despite the complexity and various natures of hematological populations, they have common characteristics. Therefore, a common methodology and tools for study designing, collection of experimental data, modeling, and analysis can be developed.