Oxidant/antioxidant imbalance has been reported in various respiratory diseases including pneumonia. The purposes of this study were the measurement of total antioxidant status in the serum of patients with community acquired pneumonia (CAP) and the investigation of the probable relationship between TAS and the severity of the disease. Thirty patients (22 males, 8 females, mean age 48±21 years) were studied. Clinical, laboratory and radiographic data obtained on admission as well as on the 7th day of the hospital stay were recorded. At the same time points, serum TAS was also determined using a colorimetric method at 600 nm. TAS values on admission (TAS1) were significantly lower compared to TAS values obtained on the 7th day of the hospital stay (TAS2) (0.84±0.13 mmol/L vs 1.00±0.17 mmol/L, respectively, P=.0001). There was a positive association between TAS1/TAS2 ratio and the change in the severity score (r=.50, P=.007). TAS1/TAS2 ratio was inversely related to PaO2 on admission (r=.47, P=.008). Furthermore, the change in TAS was positively related to white blood cell count on admission (r=.399, P=.029) and significantly greater in patients at risk for CAP (P=.01) as well as in patients with Gram(-) pneumonia (P=.032). Another interesting finding is that TAS1/TAS2 ratio was significantly higher in patients with bilateral pneumonia (P=.023). In conclusion, low TAS values in patients with CAP suggest the presence of oxidative stress. The decrease in TAS values may be useful in estimating the severity of CAP and in determining the appropriateness of antioxidant administration in selected cases. Pneumon 2002, 15(3):295-304.