Mechanisms of therapeutic action of UV blood irradiation and optimal irradiation scheduling were studied in the course of UV-irradiated blood transfusions capable of correcting lipid peroxidation (LPO) and antioxidant system (AOS) in acute pneumonia (AP) patients. Single and multiple measurements of LPO and AOS parameters (malonic dialdehyde, diene conjugates, red cell resistance to peroxide hemolysis, catalase, superoxide dismutase, cerulloplasmin, plasma total estrogens, progesterone and testosterone) were made in 10 young males with moderate AP and 20 healthy controls. UV blood irradiation in AP is shown to be pathogenetically validated. It works via effective stabilization of LPO as a result of early adequate stimulation of endogenic AOS. Positive changes were also induced in the system of hormonal regulation. It is suggested that hyperestrogenemia plays a compensatory role in AP pathogenesis.