Background. It is difficult to confirm the diagnosis of the mixed infection of the erythema migrans form of Lyme borreliosis and tick-borne encephalitis at the onset of the disease because of delayed seroconversion.
 Purpose. Assessment of hemogram predictors to develop a model for early diagnosis of the mixed infection of the erythema form of Lyme borreliosis and tick-borne encephalitis.
 Materials and methods. The study involved 21 patients with the mixed infection of the erythema form of Lyme borreliosis and tick-borne encephalitis (Group 1) and 25 patients with the erythema form of borreliosis monoinfection (Group 2), who were hospitalized no later than the 7th day of the disease. The hemogram parameters, the leukocyte intoxication index and the body resistance index were analyzed. ROC analysis was used. The logistic regression model was developed using STATISTICA 12.0 modules.
 Results. The Group 1 patients compared with Group 2 demonstrated significant increases in the levels of the leukocyte intoxication index, and neutrophils, but decreases in the levels of the body resistance index, eosinophils, basophils and platelets. The leukocyte intoxication index was a “very good” predictor of the mixed infection, and the body resistance index, the number of eosinophils, basophils, neutrophils and platelets in peripheral blood had a “good” predictive values. The model included the body resistance index, the number of eosinophils and basophils.
 Conclusion. The developed model had a “very good” predictive value for the mixed infection early diagnosis of the erythema migrans form of Lyme borreliosis and tick-borne encephalitis.