A field trial was conducted in 2002/2003 and 2003/2004 growing seasons at Giza Agricultural Research Station to evaluate the reactions of ten flax cultivars to powdery mildew (PM) disease. In general, the tested cultivars could be divided into four distinct groups, i.e., highly resistant (Ottowa 770 B, Dakota, and Bombay), resistant (Cass, Wilden, and Clay), susceptible (Koto and Marshall), and highly susceptibie (Cortland and C.l. 2008). The cultivais showed considerable variation in disease severity (DS) ranged from 3.69 on Bombay to 100% on Ci. 2008. GLC analysis of amino acid composition of cultivar seeds revealed the presence of 18 amino acids. Regarding mean percentages of the separated amino acids, glutamic add showed the highest value (2029%), while tryptophan showed the lowest value (0.45%). The other amino acids showed intermediate values between these two extremes. DS was positively correlated with serine (r= 0.858, P< 0.01). On the other hand, none of the other amino acids was significantly correlated with DS. Data for DS ratings and amounts of amino acids were entered into a computerized stepwise multiple regression analysis. Using the predictors supplied by stepwise regression, a six-factor model was constructed to predict PM severity. This model showed that PM severity differences were due largely to the amino acids serine, threonine, isoleucine, leucine, alanine and valine, which accounted for 99.83% of the total variation in DS. These results indicate that amino acid composition of linseed may provide a supplementary assay to greenhouse and field tests to distinguish quantitatively between. PM resistant and susceptible genotypes.