A novel coronavirus called the severe acute respiratory syndrome (SARS)-associated coronavirus (CoV) has been identified as the causal agent of SARS. 1-3 To understand the humoral immunity to this virus, we studied the profile of IgM and IgG antibody responses to SARSCoV. IgM and IgG antibodies were analyzed by an indirect enzyme-linked immunosorbent assay in 20 patients with SARS from week 1 of their illness to week 12 and in 103 healthy contacts. All 20 patients tested negative for IgM and IgG at week 1 after the onset of symptoms. Of these patients, 16 tested positive for IgM and 17 tested positive for IgG at week 2 (Fig. 1). All 20 patients were IgG-positive after week 3 and continued to have high levels of IgG up to three months after the onset of symptoms. The IgG titers were low at the beginning of week 2 (mean, 1:40, with the cutoff for a positive result being 1:10), increased to an average of 1:256 at week 3, and peaked at 1:640 at week 12. The IgM titers peaked during the acute or early convalescent phase and they declined with IgM disappearing by the end of week 12. All 103 healthy contacts tested negative for IgM and IgG. Our results suggest that 100 percent of patients had antibody responses to SARS-CoV during the convalescent phase. The SARS-specific IgG antibody persisted for a long time, but the SARS-specific IgM remained measurable for a much shorter period, suggesting that IgG antibody to SARSCoV may represent the primary humoral immune response protecting patients against SARS. The profile of antibodies against SARS-CoV was consistent with common findings with regard to acute viral infectious diseases such as hepatitis A. 4 The profile of anti-SARS antibodies may be helpful in the diagnosis and in epidemiologic surveys. The presence of high titers of IgG antibody to SARS-CoV in the patients at the convalescent stage also suggests that a live attenuated or inactivated vaccine could be developed for active immunization and a concentrated human SARS-specific IgG antibody for passive immunization could be developed for the treatment of SARS.