Background: Lyme borreliosis (LB) is a serious infectious disease. Carnitine plays a crucial role in metabolism and inflammatory responses. Carnitine may be important in improving neuronal dysfunction and loss of neurons.Aim: To evaluate serum carnitine concentration in adult patients with various clinical types of LB.Material/Methods: Groups: 1) patients with erythema migrans (EM, n=16), 2) neuroborreliosis (NB, n=10), 3) post-Lyme disease (PLD, n=22) and healthy controls (HC, n=32). Total (TC) and free (FC) carnitine were determined with the spectrophotometric method.Results: TC levels (44.9±10.4, 28.0±8.4, 35.9±15.6 μmol/L) in the EM, NB and PLD patients were lower than in HC (54.0±11.4 μmol/L), p < 0.001. FC levels (32.7±7.7, 23.6±6.8, 26.3±11.2 μmol/L) in the EM, NB and PLD patients were lower than in HC (40.5±7.6 μmol/L), p < 0.001. AC levels (12.2±5.2, 4.4±2.6, 9.6±7.4 μmol/L) in the EM, NB and PLD patients were lower in the NB and PLD patients than in HC (13.5±8.40 μmol/L), p <0.001. AC/FC ratio was 0.31±0.14, 0.18±0.09, 0.39±0.33 in the EM, NB and PLD patients.Conclusions: LB patients exhibit a significant decrease of their serum carnitine concentrations. The largest changes were in the NB and PLD patients. To prevent late complications of the disease a possibility of early supplementation with carnitine should be considered. Further studies are required to explain the pathophysiological significance of our findings.