The levels of some pro- and anti-inflammatory cytokines [interleukin (IL)-1β, tumor necrosis factor (TNF)-α, IL-6, IL-10, and transforming growth factor (TGF)-β], were measured by enzyme-linked immunosorbent assay (ELISA) method in the plasma of patients affected by obstructive sleep apnea syndrome (OSAS) at 22:00 hours before polysomnographic recording and immediately after the first obstructive apnea causing an SaO2 below 85%. Significantly higher levels of TNF-α were found in OSAS patients assessed before polysomnography compared with the control group (P < 0.01). A slight but significant increase in the plasma levels of IL-6 was also present (P < 0.05). Conversely, a significant decrease in the plasma levels of IL-10 was evident at baseline in OSAS patients (P < 0.04). No significant difference emerged between the mean values of IL-1α and TGF-β between OSAS patients and controls. The present data support a prevailing activation of the Th1-type cytokine pattern in OSAS patients, which is not associated with the severity and duration of OSAS. This can have important consequences for the outcome of OSAS patients, especially with regard to the increased risk for developing atherosclerosis and cardiovascular and cerebrovascular diseases. Immediately after the first obstructive apnea causing an SaO2 <85%, a significant variation was observed in the plasma levels of TNF-α in OSAS patients compared with those measured before the beginning of polysomnographic recording (P < 0.001). The role played by this further increase in TNF-α levels after the obstructive apnea in OSAS patients remains to be established in the light of the pathogenic mechanisms of this sleep disorder.