AbstractINTRODUCTION: Mediterranean spotted fever (MSF) in Bulgaria is caused by Rickettsia conorii conorii. Aim: This study aims at investigating the absolute counts of T-lymphocyte subsets in the peripheral blood of patients with MSF in order to establish relationships with disease severity. MATERIALS AND METHODS: The absolute counts of T-lymphocyte subsets were tested in the blood of 62 patients in the acute stage of MSF. They were assigned into three age and sex matched groups, based on the severity of disease - with mild, moderate or severe forms. Controls were 32 age and sex matched healthy individuals. The diagnosis was confirmed by an immunofluorescence assay. Immunophenotyping was performed using Epics XL-MCL Coulter, USA flow-cytometer. RESULTS: The absolute counts of immune competent (CD3+) cells, as well as the counts of helper/inducer (CD3+CD4+) and suppressor/ cytotoxic (CD3+CD8+) T-cell subsets decreased in parallel with disease severity. Naïve (CD4+CD45RA+) and activated memory (CD4+CD45RO+) T-cell subsets were reduced, particularly in severe MSF. Taken as a whole, the counts of activated (CD3+HLA-DR+) and that of presenting accessory (CD28+) or stimulatory (CD38+) molecules Т-cell subsets was increased, but in the first two subsets the trend from mild to severe forms of the disease was descending. CONCLUSION: Reduced T-lymphocyte subset counts are likely related to trans-migration into perivascular inflammatory foci. The increased number of T-lymphocytes bearing activation molecules reflects a mobilization of the cell-mediated immune response. An important issue of this study is the possible prognostic value of T-cell subsets counting, predicting the evolution of a clinical condition to clinical forms, according to the disease severity.