A total of 613 eligible index case-patients, detected from May 16 through September 15, 2009, were included in this study. Through fi eld epidemiologic investigations, 7,099 close contacts were traced and quarantined in Beijing. The median number of close contacts per index case per day was 7.0 persons (range 2.0-95.0 persons); the median number for an imported index case was 7.0 persons (range 1.7-95.0 persons) and for a locally acquired index case was 5.3 persons (range 1.0-25.0 persons). For the 601 symptomatic index case-patients, the median interval between illness onset and sample collection was 1.0 days (range −1.9 to 7.0 days).