Mediterranean spotted fever (MSF) in Bulgaria shows increasing severity, especially in patients over sixty years. As T lymphocytes are the primary effector cells in MSF and are altered by the ageing process we aimed to specify the changes in their numbers in elderly patients. We performed a clinicoepidemiological study, haematological and biochemical analysis of 132 patients of 60+ and 30 patients between 19 and 57 years with MSF. We investigated the cell immunity of 20 patients of 60+ using immunofluocytometry. The control group consisted of 10 younger patients and 10 healthy individuals of 60+. MSF showed more unfavourable course in the elderly.We found T-lymphocytes depletion in all patients with MSF. Activated T lymphocytes were increased in elderly patients with MSF. We found significant differences between the number of activated T lymphocytes in elderly patients and healthy elderly persons. The observed T-lymphocytes depletion in MSF corresponded to the disease severity. The increase in the activated T lymphocytes in patients over 60 years contributes to an adequate immune response., Margarita Gospodinova, and Literatura