The accumulated data indicates that asthma in associated with low serum Selenium level . Selenium is an essential part of the enzyme GSH-Px which acts as free radical scavenger . GSH-Px catalyses the reaction of I^Oj and other organic hydroperoxides into alcohol and water. In asthmatic patients there is generation of free radicals as a part of the inflammatory process, and patients with FEV1% of less than 80% have a higher spontaneous superoxide production, and the generation of superoxide is inversely correlated with FEV1%. The aim of this study was to asses the changes in serum Se level with the changes in FEV1 and VC (parameters used to asses the severity of asthma) as well as to study the correlation between these parameters and the serum Se level. The study was carried out on 172 subjects, (108 normal and 64 asthmatic patients), their age ranged between 15 to 60 year. In this study all subjects (patients and control) were tested for pulmonary function test using a computerized spirometer. Blood samples were taken from all the subjects for serum Se estimation. In asthmatic patients the P.F.T was repeated four times in different days following the acute attack of asthma: (day 1, day 4, day 7 and day 14) and blood samples were taken immediately following the test. The serum Se level was measured using Atomic Absorption Spectrophotometer based on direct thermal atomization. Our results showed a significant difference in FEV1, VC and serum Se level between control group and asthmatic patients in different days following the acute attack of asthma p<0.0001. Our results also showed a positive correlation between changes in the serum Se level and the severity of asthma, were the correlation coefficient between serum Se level and FEV1 in different days following the acute attack of asthma was statistically significant r=0.288 P< 0.01. We also found a positive correlation between serum Se level and VC were the correlation coefficient between the serum Se level and VC was statistically significant r=0.369 P < 0.01. The comparison by analysis of variance showed a significant difference in FEV1, VC and serum Se level between the different days following the acute attack of asthma and the control group p<0.000.1. From this study we made a conclusion that the changes in serum Selenium level correlate with the severity of asthma.