AIM: To study the affecting factors on the production of antibodies in SARS patients. METHODS: In a total of 104 clinically diagnosed SARS patients, the relations among IgG antibody titer, patients age, duration and the dosage of cortiscosteroids therapy were investigated. RESULTS: Eighty-three (79.8%) patients were positive for SARS-Cov antibody. The range of time for the occurrence of the antibody was 8 to 32 days; the percentage for the occurrence of the antibody in the periods of 0-7, 8-14, 22-28, 29-35, 36-42, 43-56 and 57-63 d were 0, 50% (21/42, 63.6%) 42/62, 73.3% (44/60), 88.1%(37/42), 81%(17/21), 92.6% (25/27) and 50% (2/4) respectively. The titer of antibody also increased from 0.14±0.05 in 0-7 d to 0.69±0.10 (43-56 d) and started to reduce after 8 wk. The positive percentage and titers of antibody grew when the dosage of cortiscosteroids was added. The positive percentage of antibody in severe patients and mild patients were 95.7% (22/23) and 75.3% (61/81) respectively, and the titers of antibody in the severe patients were higher than that of the mild patients (P >0.05) and the same change was also found between 19-14 age group and 46-52 age group. CONCLUSION: 79.8% of SARS patients are positive for SARS-Cov antibody. The positive percentage and the titer of antibody grow gradually while the duration prolongs, but declines after 8 wk. It is positively related to the clinical condition, but not affected by the dosage of cortiscosteroids and the age of the patient.