The aim of this histological study was to analyze the gingival reaction to fixed orthodontic appliances. Gingival specimens were obtained with minimal trauma from 11 patients treated with fixed appliances in different intervals during the orthodontic treatment, including post-treatment periods. Serial sections were stained with Hematoxylin-Eosin. T- and B-cells were identified by specific antibodies, using a double staining technique with Avidin-Biotin method. Histological observations demonstrated and confirmed the presence of gingivitis during orthodontic treatment. According to the usual histological evaluation, the biopsies revealed the presence of hyperplastic chronic inflammatory changes from mild to moderate severity. The lack of rapid increase of CD20+ cells demonstrated that the gingival inflammation did not cause overall tissue destruction.