Oxidative stress due to increased production of oxygen free radicals has been reported in various respiratory diseases including asthma and pneumonia. An imbalance between oxidant and antioxidant production has also been proposed in these patients. In the present study, serum total antioxidant status (TAS) was measured in patients with severe exacerbation of bronchial asthma or community-acquired pneumonia; a possible correlation between TAS and disease severity was investigated. Twenty patients (10 men, 10 women; mean age 41±20 years) admitted to hospital for severe exacerbation of asthma and thirty patients (22 men, 8 women; mean age 48±21 years) with community-acquired pneumonia were studied. Ten healthy non-smokers (44±16 years of age) were also included in the study. On days 1 and 7, serum total antioxidant status was measured using a colorimetric method in 600 nm. At the same time, clinical and laboratory severity criteria were recorded in both groups of patients. In the first measurement, TAS values were found statistically significantly lower in both asthma and pneumonia patients compared to normal subjects (0.98±0.08 vs 1.19±0.09 mmol/L, p<0.001; and 0.84±0.13 vs 1.19±0.09 mmol/L, p<0.001, respectively). TAS values obtained on day 1 were also lower compared to the respective values on day 7 in both groups of patients (0.98±0.08 vs 1.12±0.17 mmol/L, p<0.001; 0.84±0.13 vs 1.00±0.17 mmol/L, p=0.0001, respectively). In asthmatic patients, TAS changes correlated with FEV1 changes (r=0.58, p=0.007). In pneumonia patients, TAS changes were associated with factors predisposing to pneumonia (p>0.001), complications (p=0.005), and pneumonia caused by Gram(-) bacteria (p=0.008). Additionally, TAS changes correlated with white blood count (r=0.39, p=0.03), especially polymorphonuclear cell count (r=0.36, p=0.05). Finally, the comparison between TAS values in asthma and pneumonia showed statistically significantly lower values in pneumonia in both measurements (0.84±0.13 vs 0.98±0.08 mmol/L, p<0.001; 1.00±0.17 vs 1.12±0.17 mmol/L, p<0.001, respectively). We concluded that serum total antioxidant status in patients with severe exacerbation of bronchial asthma or community-acquired pneumonia is relatively low at the onset of the disease. In the course of the disease, an increase in total antioxidant status is consistent with clinical and laboratory improvement. Total antioxidant status changes are associated with respective changes in parameters related to disease severity in both asthma and pneumonia patients. Pneumon 2005, 18(3):315-324.