BACKGROUND. Autoimmune lymphoproliferative syndrome (ALPS) is a primary immunodeficiency disorder characterized by immune dysregulation. ALPS is defined as chronic, nonmalignant lymphoproliferation in patients with double-negative T cell (DNT) expansion over 1% and defective lymphocyte apoptosis. The etiology of the disease has not been fully explained; the spectrum of its clinical manifestations is broad and the exact diagnostic criteria have not yet been established. AIM. Our goal was to define the clinical, immunological and genetic background of ALPS patients in our country. METHODS. The study included all patients under suspicion of ALPS treated at the University Children’s Hospital in Ljubljana. The criteria established by the investigators were: age under 18 years and presence of at least one clinical manifestation and at least one immunological finding characteristic for ALPS. The study was retrospective and involved a review of clinical documentation. Information about clinical, immunological and laboratory findings was collected for every patient, from the time of first presentation of the disease. The patients were followed up to the last clinical examination. At the Institute of Microbiology and Immunology, lymphocyte subtypes were quantified by flow cytometry, serum values of IL-10 were evaluated using the ELISA method, and apoptotic cell death was assessed. At the genetic laboratory, the nucleotide sequences of the Fas gene which encodes Fas protein and Fas ligand gene were analysed. RESULTS. The study included 11 patients. Three of them fulfilled all diagnostic criteria for ALPS. The other 8 patients fulfilled the criteria determined by the investigators. This means that they had one clinical and one laboratory finding characteristic for ALPS, but did not fulfil all of the criteria needed for the diagnosis of ALPS. CONCLUSION. ALPS is a variable clinical condition with manifestations that can present later in life. The characteristic immunological findings include 314 elevated serum values of IL-10, the presence of DNT cells and defective lymphocyte apoptosis in vitro. Not all tests are positive in every patient and for this reason a whole series of tests should be made. In our study, genetic analyses revealed that the most common mutation in Slovenia is not in the gene encoding Fas protein. A protocol of diagnostic procedures was also developed to be used whenever ALPS is suspected.