The objective of the study was evaluation of the frequency of occurrence of coinfections with Borrelia burgdorferi and other microorganisms transmitted by ticks: Anaplasma phagocytophilum, Babesia spp. and Bartonella spp. in patients with the diagnosis of borreliosis. The study covered 160 patients referred to serologic tests by the Outpatient Departments at the Institute of Agricultural Medicine in Lublin and 50 blood donors who constituted a control group. Plasma was examined for Borrelia IgM and IgG antibodies with the use of immunoenzymatic tests ELISA (Bellco Biomedica, Austria), whereas for anaplasmosis, babesiosis and bartonellosis – by the indirect immunofluorescence test IFA IgG (Focus Technologies, USA). Statistical analysis of the results obtained was performed by means of χ2 test; the p values p>0.05 were considered statistically significant. In the group of patients analysed, antibodies against Borrelia burgdorferi were detected in 104 people (65,0%). In the case of 126 (78.7%) patients antibodies against Bartonella sp. were detected, in 38 cases (23.7%) antibodies against Babesia microt, were found, and antibodies against Anaplasma phagocytophilum were detected in 34 patients (21.2%). In the control group, 4 positive results were noted with Borrelia burgdorferi antigen, 12 with Bartonella sp. antigen, and 3 positive results for Anaplasma phagocytophilum, while no antibodies against Babesia microti were observed. The results of the study show a frequent co-occurrence with borreliosis of infections in humans with other pathogens transmitted by ticks, which creates a possibility of prognosticating clinical procedure adequately adjusted to the state of the disease. In a patient with suspicion of borreliosis, tests should be performed for the presence of other tick-borne pathogenic agents in order to makea precise diagnosis and undertake proper treatment.