Modern concepts on the etiology and pathogenesis of malignant tumors are reported, two basic links being distinguished-genetic factors and environmental factors--modifying ones. The specific weight of both factors in the etiology and pathogenesis of different oncological affections is varied. Based on the analysis of the modern concepts of the origin and development of a tumor the authors have elaborated a working scheme of selection of oncological risk groups among the population. The modifying factors being of primary importance for tumor growth are grouped by the authors into exogenous and endogenous ones. It is suggested to judge the degree of their manifestation in different individuals by the duration of their effect. The working scheme concerned may provide a logical basis for assessment of risk groups in various oncological lesions, but it needs some further verification in clinical practice.