Objective To compare the TCM syndrome characteristics of Type A H1N1 Flu with those of the ordinary seasonal flu in the same year in order to guide clinical diagnosis and treatment. Methods The data of 200 cases of Type A H1N1 Flu and 200 cases of ordinary seasonal flu in the same year were collected. The cross section investigation on the general clinical manifestations and TCM syndromes of Type A H1N1 Flu cases was carried out and compared with those of ordinary seasonal flu cases for analysis. Results The average body temperature of Type A H1N1 Flu group was significantly higher than that of the ordinary seasonal flu group （P<0.05）. There was statistical difference between the two groups in the frequency of symptoms including fever

sore throat

heaviness in extremities

heaviness in the head

aversion to wind and cold

chills

headache

obstruction of nose

running nose

and thirst （P<0.01）. With the seasonal change

in the Type A H1N1 Flu group

the rate of heaviness in the head

heaviness in extremities

aversion to wind and cold

and chills increased. There was statistical significance in summer and autumn as compared with winter （P<0.05 or P<0.01）. Conclusion The causative factor of Type A H1N1 Flu is the pathogenic heat combined with damp

and its pathogenesis is that the damp-heat invades the lung

causing a pathogenic condition in which the lung and defensive qi are both diseased

therefore

the exterior syndrome is so short and the pathogen enters the interior so quickly.