SUMMARY. BACKGROUND: Several lines of evidence suggest immune system derangement in patients with Obstructive Sleep Apnoea Syndrome (OSAS), expressed by elevation in the circulating levels of proinflammatory markers and increase in the total number of neutrophils, but there is less information on possible alterations in lymphocyte expression. OBJECTIVES: The aim of this study was to explore the differences between OSAS patients without comorbidities and healthy control subjects in lymphocyte subsets and in apoptosis. PATIENTS AND METHODS: In 12 otherwise healthy OSAS patients (AHI≥5/h+symptoms) and 12 age- and body mass index (BMI)- matched, healthy control subjects (AHI<5/h) layers of mononuclear cells from were isolated from blood samples following a density gradient centrifugation with Ficoll-Histopaque. The quantification of lymphocyte subsets was carried out by whole blood multicolour flow cytometry. Spontaneous early apoptosis was also assessed by flow cytometry after simultaneous staining with Annexin V and various markers for lymphocytic subsets. Annexin V expression is a characteristic of the early apoptosis of whole blood leucocytes. RESULTS: The absolute number of Large Granular Lymphocytes (T-LGLs) was significantly lower in OSAS patients than in the healthy control subjects. Conversely, the percentage and the absolute numbers of the other lymphocyte subsets tested (CD4+, CD8+, CD19+, ΝΚ and γδ cells) showed no significant difference between patients with OSAS and healthy subjects. No difference was observed in early apoptosis of the above lymphocyte subsets between OSAS patients and control subjects. CONCLUSIONS: The spontaneous apoptosis of peripheral blood lymphocytes is not altered in OSAS patients compared to non-apnoeic individuals. However, the significant reduction of T-LGLs in the peripheral blood of OSAS patients supports the concept of immune derangement in OSAS, as such a decrease has been observed only in a few autoimmune diseases. Pneumon 2009; 22(1):–