The results of study of 36 children’s clinical histories with referral diagnosis of “long-lasting subfebrility of unidentified genesis” are given in this article. Subfebrility occurs more often in mid-childhood children, who belong to the group of frequently ill children and it is characterized by monothermia in 82.4% of cases. The main subjective sign of a subfebrile condition is “chronic fatigue” syndrome. Laboratory features of subfebrile syndrome are: anemia of the 1st degree, neutrophylia due to the segment-nuclear forms, moderate monocytopenia, and slight increase of ESR. In 94.5% of cases the changes for herpetic infection using ELISA test were detected, that indicated previous infection or virus persistence of varying degree of activity and was argued by the increase of general serum IgG level. The key reasons of the fever of uncertain origin became: in 72.99% of cases – bacterial polytopic infections (in general infections of a biliary system), in 18.9% of cases – active persistent herpetic infections with lymphadenopathy, in 8.11% of cases – vegetative dysfunction. Analysis of procalcitonine level in patients with fever of unknown origin indicates, that it can be used as a marker of clinical visible bacterial infection and can confirm non-viral genesis of the probable disease. Long-lasting subfebrile conditions have no self-meaning and are related to latent foci of infection, persistent virusemia with lymphadenopathy signs