Abstract: Objective To analyze the reporting of unknown etiology pneumonia (UEP) cases from 2004 to 2009 in China and explore the possible influencing factors. Methods The analysis was conducted on the intervals between illness onset and diagnosing, diagnosing and reporting as well as reporting and verifying of the UEP cases reported through passive reporting surveillance system during this period. Results A total of 864 cases were reported from 2004 to 2009, the reported incidence peaked during the SARS epidemic in Beijing and Anhui from April to May in 2004, and during the epidemics of human avian influenza in winter of 2005 and spring of 2006 and in early 2009. The medians of interval between illness onset and diagnosing, diagnosing and reporting as well as reporting and verifying were 5.0 days, 0.7 day and 0.1 day respectively. The median of interval (8.0 days) between illness onset and diagnosing of UEP cases reported during the two epidemic peak periods of human avian influenza were longer than that in other period. The median of interval (8.0 days) between illness onset and diagnosing of the UEP cases which were diagnosed as human avian influenza was longer than that of the UEP cases which werent diagnosed as human avian influenza. Conclusion Though the UEP surveillance played an important role in the case finding of human avian influenza, the problems still exist. According to the surveillance situation, it is suggested that the case definition of UEP and surveillance procedure should be revised to improve the surveillance.