Over the past decades, the incidence of brain tumors in Russia has no tendency to decline and is 2.4–2.6 cases per 100 000 children. At present, 1000–1200 new cases of brain tumors are registered annually in Russia. Purpose of the study. Based on the complex analysis of toxin formation, free radical oxidation and mechanisms of their neutralization, to reveal the pathogenetic factors of the progression of the severity of the general condition of children with medulloblastoma of the cerebellum against chemoradiation therapy. Material and methods. The pathogenetic factors of the progression of the severity of the general condition of children (n=40) with medulloblastoma of the cerebellum on the background of chemoradiotherapy have been studied. The duration of the study was 60 days. Results. It has been established that one of the most important pathogenetic factors of the increase in the severity of the general condition of children with medulloblastoma of the cerebellum against chemoradiotherapy is the activation of free radical oxidation processes, accompanied by an increase in the content of lipoperoxidation products in plasma and urine by 20–40%. Another pathogenetic factor of the increase in the severity of the general condition is a significant increase in the plasma and urine content of low and medium molecular weight substances responsible for the intensification of free radical oxidation processes, as well as for the formation of a vicious circle: activation of free radical oxidation — membrane destruction — endotoxicosis. Conclusions. The failure of compensatory capabilities of detoxification organs was revealed, interrelations of changes in the intensity of free radical oxidation, membrane destruction and toxin formation in children with medulloblastoma of the cerebellum, who received standard chemotherapy, were established.