Profiles of antibodies to the nucleocapsid protein of the severe acute respiratory syndrome (SARS)-associated coronavirus in 445 probable SARS patients and 3,749 healthy people or non-SARS patients were analyzed by antigen-capturing enzyme-linked immunosorbent assay. Antinucleocapsid antibodies were elucidated in 17.5% of the probable SARS patients 1 to 7 days after the onset of symptoms and in 80% of the patients 8 to 14 days after the onset. About 90% of the probable SARS patients were positive 15 or more days after illness. Antibody titers increased up to 70 days, and high antibody titers were maintained at least for another 3 months. Of the healthy people and non-SARS patients, only seven (0.187%) were weakly positive.