Background: Diagnosis of drug allergy is difficult because few methods have been validated in the literature. In the last few years, identification of T cell activation markers to assess drug allergy has been the focus of several studies. Objective: The aim of the present work was to search for CD25 and CD69 markers on T CD4+ and T CD8+ cells in drug allergy. Methods: Fourteen patients with drug hypersensitivity were enrolled in this investigation. Some patients had at least one adverse reaction to one or more suspected drugs, therefore, a total of 16 reactions and 10 drugs were investigated. Prick or patch tests were done according with the time of onset and type of the clinical manifestations. In vitro studies were performed by incubating peripheral blood mononuclear cells from patients and controls with different concentrations of the suspicious drugs for 72 hours. The samples were stained with fluorochrome- labelled monoclonal antibodies against CD69, CD25, CD4 and CD8 molecules and analyzed by flow cytometry. Results: Statistical differences were found at medium and high drug concentrations for the CD4+CD69+ marker (p ≤ 0.05), at the lowest drug concentration for the CD4+CD25+ and CD8+CD69+ markers (p ≤ 0.05) and at the highest drug concentration for the CD8+CD25+ marker (p
