Objective. To study an immunological profile at prodromal and manifesting stages of endogenous juvenile psychosis. Methods. Authors studied 77 patients, aged from 16 to 25 years. Patients were stratified into 2 groups. The first group included 39 patients without psychotic symptoms (prodromal group) and the second one - 38 patients with the first episode of psychosis (psychotic group). A complex of immunological parameters included phagocyte activity, cytotoxic activity, natural killer lymphocytes, production of interleukins (IL-1, IL-4, IL-10 and -interferon), content of circulating immune complexes. These parameters were measured at baseline and after treatment. Results. The changes in the immune system emerged at the very early stages of the disease. There was the activation of proinflammatory and anti-inflammatory interleukins, decrease in the phagocyte index and cytotoxic activity of natural killer lymphocytes. The high activity of IL-10 production, decreased levels of the cytotoxic activity of natural killer lymphocytes and reduced phagocyte index as well as the accumulation of circulating immune complexes are maintained during psychosis. Conclusions. The authors recommend to use immunomodulating drugs, along with psychotropics, at the very early stages of the disease to prevent poor outcome.