The aim of this study was to quantitate total T lymphocytes (total CD3+ cells) and T-lymphocyte subtypes (CD4+ [T helper] and CD8+ [T suppressor] cells) in patients with chronic sinusitis who were treated with functional endoscopic sinus surgery (FESS) and to investigate the pathophysiology of persistent inflammation in chronic sinusitis. This prospective study was conducted in study and control groups. The study group consisted of 32 patients (20 male, 12 female) with chronic sinusitis who underwent FESS. The control group consisted of 8 nonsinusitis patients (5 male, 3 female) who underwent septoplasty. Specimens from the study group were excised from five regions: the uncinate process, maxillary and ethmoid sinuses, and middle and inferior turbinates. The specimens were examined with x10 magnification by light microscopy, and the slides with a severe inflammatory process were included. Punch biopsy of the control group was taken from the inferior turbinate with patients' written approval. The surgical specimens from the study and control groups were examined with an immunohistochemical staining technique with monoclonal antibodies against CD3, CD4, and CD8 surface antigens of T lymphocytes. In every specimen, the numbers of CD3+, CD4+, and CD8+ cells were calculated in 3 to 4 high magnification field on light microscopy, and the mean number of these cells in the epithelium, subepithelial layer of the lamina propria, and deep paraglandular layer of the mucosa was determined. Statistical analysis by Kruskal-Wallis analysis of variance and the Mann-Whitney U test with Bonferroni correction revealed that the CD3 epithelial layer value of the inferior turbinate (p = .030) and the CD4 deep layer value of the middle turbinate (p = .048) were significantly higher than the corresponding values of the control group. In the epithelial (p = .018) and subepithelial (p = .012) layers of the uncinate process group, in the epithelial (p = .050) and subepithelial (p = .012) layers of the ethmoid sinus group, and in the subepithelial (p = .018) and deep paraglandular (p = .012) layers of the middle turbinate group, the difference between the CD4+ and CD8+ cell counts was found to be statistically significant by the Wilcoxon signed rank test. The number of CD4+ cells was higher than the number of CD8+ cells. In conclusion, T cells play a role in the pathophysiology of chronic sinusitis. CD4+ T helper cells, in particular, are predominant at the initiation and regulation of inflammation. The uncinate process, ethmoid sinus, and middle and inferior turbinates have the main roles by T cells and subtypes in the defense system in chronic sinusitis.