We analyzed the relationship between infection and its activation on the one hand and the effectiveness of the treatment of uveitis with genetically engineered biological drugs on the other hand. The study included 69 children presenting with endogenous uveitis. The serological markers of chronic and acute infections were detected before and during the period of up to 5 years after the onset of therapy (184 serum samples). It was shown that prior to the prescription of the genetically engineered biological drugs, infection with herpes viruses was documented in 92.6% of the children. Chronic type 1 and 2 herpes simplex infection was confirmed in 55.5% of the patients,cytomegalovirus infection in 63%, and Epstein-Barr virus infection in 88.9%. One or several markers of active infection were identified in 42.1% of the infected subjects. Infection with herpes simplex virus occurred more frequently than that with Epstein-Barr virus or cytomegalovirus (41.7% and 11.1%, and 7.7% respectively). Seropositivity for pathogens was documented only in a few cases, viz. 11.1% of the patients were infected with mycoplasma and 11.1% with ureaplasma, 7.4% with Chlamydophila pneumonia and 3.7% with toxoplasma. Only 7.4% of the children included in the study were totally free from infection with the above pathogens. The patients treated with genetically engineered biological drugs tended to be more frequently infected than before therapy. Specifically, infection with cytomegalovirus increased from 63% to 88.9 and with Epstein-Barr virus from 88.9% to 100%. Moreover, the occurrence of serological markers of activation of herpes simplex virus-1 increased from 22.2% to 44.4% and those of activation of herpes simplex virus-2 from 3.7% to 22.2%. The analysis of seropositivity dynamics separately for different pathogens demonstrated the presence of type 1 and 2 herpes simplex viruses in 10.5% of the patients and cytomegalovirus in 25% of the previously seronegative patients, the markers of activation of herpes simplex virus-1, herpes simplex virus-2, cytomegalovirus, and Epstein-Barr virus appeared in 50%, 18.2%, 5%, and 11.5% of the chronically infected children respectively without their detection. Active uveitis was diagnosed in 62.9% of the children infected with one or several herpes viruses during the period when they showed up infection activation markers and in 73.1% of the children having no such markers. The occurrence of such markers in the uninfected children amounted to 90% (p > 0.05). The results of the present study indicate that the children treated with the genetically engineered biological drugs had both primary herpes virus infection and the newly appearing markers of infection activation which can be attributed to the immunosuppressive action of these preparations. Despite the absence of the clear-cut relationship between uveitis activity and the presence of the above serological markers, the growing seropositivity rate and the frequency of the virus activation markers reflect the potential risk of manifestations of the subclinical infection. It suggests the necessity of its laboratory monitoring and further investigations.