In a cohort of 38 patients with severe acute respiratory syndrome (SARS), we observed leukopenia in 47% of patients, lymphopenia in 84%, and T lymphopenia in 95%. CD4(+) T lymphocyte levels were reduced in 100% of patients, CD8(+) T lymphocyte levels were reduced in 87%, B lymphocyte levels were reduced in 76%, and natural killer cell levels were reduced in 55%. Our data suggested that these patients' immune systems were impaired during the course of SARS. The absolute counts of lymphocyte subsets demonstrated a clinical significance for patients with SARS.