STUDY OBJECTIVES: Several lines of evidence suggest immune system derangement in obstructive sleep apnea syndrome (OSAS) patients. However, no data exist on the long-term effect of continuous positive airway pressure (CPAP) treatment on systemic immunity. Hence, we sought to evaluate this effect on various immunological parameters in OSAS patients.DESIGN: Prospective case series.SETTING: Sleep unit of a general hospital.PATIENTS: Newly-diagnosed, nonsmoking, otherwise healthy OSAS male patients (n = 52) were evaluated on diagnosis and 6 months after CPAP treatment. According to compliance to CPAP use at 6-month follow-up, they were divided into 2 groups: group A (n = 32): good compliance (mean CPAP use > or = 4 h/night); and group B (n = 20): poor compliance (mean CPAP use < 4 h/night).INTERVENTIONS: N/A.MEASUREMENTS AND RESULTS: Blood samples were obtained at baseline and at the 6-month follow-up. Percentage and absolute count of lymphocyte subsets (by flow cytometry), serum TNF-alpha, IL-6, and uric acid levels were measured. No differences were recorded regarding the baseline anthropometric or sleep characteristics of the 2 groups. In group A, a significant decrease in the absolute count of total lymphocytes (P = 0.003), and of CD4+ cells (P = 0.001), and a decrease in TNF-alpha levels (P = 0.001) and uric acid levels (P < 0.001) was observed after CPAP application. On the contrary, no alterations occurred in any of the tested parameters in group B patients.CONCLUSIONS: The selective reduction of soluble and cellular immune response factors only in those OSAS patients who exhibited good compliance to CPAP therapy provides further evidence for an ongoing systemic immune process in OSAS.