OBJECTIVE: The aim of this study was to determine the correlation between the number of FOXP3 + T cell in lesions and the disease activity of patients with oral lichen planus (OLP). MATERIALS AND METHODS: The expression of FOXP3 was investigated using immunohistochemical staining and real-time RT-PCR in 23 OLP lesions and 12 controls. Changes of FOXP3 + Treg in peripheral blood from three patients' pre and post-treatment were assessed using flow cytometry. RESULTS: Few FOXP3 + cells were detected in controls, but an increased number of FOXP3 + cells were observed in lesions (n = 20, 40.99 ± 24.68 cells per high-power field -hpf). Furthermore, the frequency of FOXP3 + Treg in reticular OLP (n = 7, 63.6 ± 23.2 cells per hpf) was significantly higher than that in erythematous ⁄ erosive OLP (n = 13, 28.8 ± 16.8 cells per hpf, P = 0.001). In addition, negative correlation was found between the number of FOXP3 + Treg and disease activity (correlation oefficient = )0.557, P = 0.013). The proportion of FOXP3 + Treg showed remarkable increase in peripheral blood from patients after treatment (1.39 ± 0.71% vs 4.91 ± 1.59%). CONCLUSIONS: These data indicated that FOXP3 + Treg were involved in the pathogenesis of OLP and correlated with disease's subtype and activity. Oral Diseases (2010) 16, 76-82