To detect correlations between changes in interferon (INF) system and a course of chronic pyelonephritis (CP), to ascertain a corrective effect of cyclopheron in combined treatment with antibacterial drugs.We examined 30 CP patients in different periods of the disease: group 1 had latent CP without laboratory evidence for exacerbation, group 2 had CP exacerbation treated with antibiotics, group 3 had exacerbation given basic treatment and cyclopheron by a standard scheme. All the patients were examined for interferon status, patients of groups 2 and 3 were examined once more after therapy.Patients of all the groups showed diminished ability of blood leukocytes to produce IFN, especially IFN-gamma. Cyclopheron addition to antibacterial therapy significantly stimulated production of IFN alpha and gamma by blood cells compared to basic therapy. Examination of 5 patients after 1-year follow-up detected preserved ability of blood cells to produce IFN and reduce the number of exacerbations after combined treatment with cyclopheron.Administration of cyclopheron in combination with standard antibacterial treatment results in persistent correction of IFN status.