Objective To detect IgG and IgM antibodies in the sera of SARS patients, in order to find the possible causative association between new coronavirus and SARS; to compare IgG and IgM antibody titers of acute phase with those of convalescent phase, in order to find specific serological reaction of SARS virus and its clinical significance. Methods Clinical specimens of SARS patients in 4 hospitals of Guangdong Province were collected and then detected by IFA and ELISA. The specificity and cross-reaction of each specimen for confirming the new coronavirus were evaluated. Results One hundred and seventeen of 130 clinically diagnosed SARS patients showed viral specific serological reaction, with the positive rate of 90%. The IgG titer increased significantly 10 days after the onset of infection and reached a higher level (above 320) 15 days later, while IgG titer reached the highest level on the 20 th-30 th day. The specific antibodies were negative in all 119 individuals with close contact with patients and 100 normal controls. Conclusion Viral specific IgG antibody was detected in the sera of SARS patients, confirming that the patients were infected by the virus; persistent high titer of IgG antibody in the sera of convalescent patients might probably be a protective mechanism in general population.