Objective To study the TCM syndromes and etiology-pathogenesis of severe and critical cases of Influenza A （H1N1）.Methods One hundred and twenty-three patients were classified into severe case group （59 cases） and critical case group （64 cases） based on the diagnostic criterion.Their course of disease

onset of disease

symptom frequency

appearing time

and disappearing （or significantly relieving） time were described respectively and the symptomatology and development law of TCM syndromes were cleared up and analyzed.Results The onset of disease of severe and critical cases was mostly occurred in the season of late autumn or winter.Fever

cough

and expectoration were the main symptoms of severe cases

while chills

fever

cough

expectoration

hemoptysis

fullness and suffocation feeling in chest

fatigue

and shortness of breath were the main of critical cases.The course of disease and lasting time of all symptoms in the critical cases were generally longer than those in the severe cases.Conclusion Influenza A （H1N1） is divided into four stages:initial

advanced

extreme

and recovery.Its initiating agent is evil heat and pestilential toxin with a core pathogenesis as toxic heat blocking in the lung

toxic heat combined with blood stasis

and lung qi obstructing.Its development law of TCM syndromes comes from toxic heat blocking the lung

to toxic heat damaging the collaterals of lung

to dyspnea

to cold limbs

and to exhaustion of both qi and yin.Its pathological change takes the febrile disease with lung and stomach involved and with qi-system injured as the important condition.