Leukoses - the tumour of hemopoietic system arising from hemopoietic cells and primarily amazing an osteal brain. Separation of leukoses into two basic groups - acute and chronic - is defined by a structure of tumoral cells: to acute leukoses which cellular substrate is presented by blasts, and to chronic - leukoses at which the great bulk of tumoral cells consists mainly of mature elements are carried. A principal cause of occurrence of leukoses are chromosomal aberrations. The most widespread mutagen factors: an ionizing radiation, influence of the certain chemical substances (bond of benzene, cytostatic preparations - Imuranum, Cyclophosphanum, Leukeranum, Melphalanum, Mustargenum). The probability of occurrence of acute leukoses among the patients received cytostatic therapy (so-called secondary leukoses), raises in hundreds times. The interval before occurrence of a leukosis after influence of an ionizing radiation makes 5-10 years, and after a chemotherapy - 2 years with a maximum of 6-10 years.