Objective To investigate the changes of subgroups of peripheral blood T lymphocytes in patients with severe acute respiratory syndrome (SARS) and its clinical significance.Methods Subgroups of blood T lymphocytes in 93 patients with SARS were detected by flow cytometer. The results detected in 64 normal subjects and 50 patients with AIDS served as controls.Results The numbers of CD3 +, CD4 +, and CD8 + lymphocytes all significantly decreased in acute phase of patients with SARS [(722±533)/μl, (438 ±353)/μl, (307±217)/μl)] compared with those in normal controls [ (1527±470)/μl, (787±257)/μl, (633±280)/μl, all P<0. 01) ], which was different from what we observed in patients with AIDS who had decreased CD4 + [ (296±298)/μl] but increased CD8 + [ (818 ±566)/μl counts. The counts of CD3+, CD4+, and CD8 + lymphocytes decreased more apparently in patients with severe SARS. All the five patients who died had CD4 + counts less than 200/μl. As the patients’ condition improved, CD3 +, CD4 +, and Objective To investigate the changes of subgroups of peripheral blood T lymphocytes in patients with severe acute respiratory syndrome (SARS) and its clinical significance. Methods Subgroups of blood T lymphocytes in 93 patients with SARS were detected by flow cytometer. The results detected in 64 normal subjects and 50 patients with AIDS served as controls. Results The numbers of CD3 +, CD4 +, and CD8 + lymphocytes all significantly decreased in acute phase of patients with SARS [(722 ± 533) / μl, (438 ± 353) / μl (307 ± 217) / μl) compared to those in normal controls [(1527 ± 470) / μl, (787 ± 257) / μl, (633 ± 280) / μl, all P <0.01) which was different from what we observed in patients with AIDS who had decreased CD4 + [(296 ± 298) / μl] but increased CD8 + [(818 ± 566) / μl counts. decreased more apparently in patients with severe SARS. All the five patients who died had CD4 + counts less than 200 / μl. As the patients’ condition improved, CD3 +, CD4 +, and