Reiter's disease in children was shown to be not such a rare disorder and to occur as the polysystemic process. The inflammation is induced by trigger agent (mostly by Chlamydia trachomatis), but then afterwards the key role has inadequate activation of cell-mediated and humoral immunity that may be manifested by substantial enhancement of leuko- and lymphopoiesis. There is an elevation of total number of leukocytes, T cells and lymphocytes, expressing CD4, CD8 and CD25 receptors with sharp increase of CD4/CD8 ratio. The numbers of natural killer cells (CD16) and cells, expressing activation markers, are 2 times higher than in healthy children. Besides, levels of IL-4 and IFN-gamma were increased in great numbers. Severity and duration of the disease course are genetically predetermined. Children - carriers of HLA B27 antigen - had 10-fold elevation of CD71 cells and IFN-gamma in comparison with control group and patients with Reiter's disease, but without this antigen.