Folliculitis decalvans (FD) is a chronic neutrophilic scarring alopecia, the exact pathogenesis of which remains unknown. A recent report on the successful use of a novel anti-interleukin (IL)-17 biologic agent to treat FD and the presence of IL-17-secreting mast cells in other types of scarring alopecias point to a distinct pathogenic mechanism in this disease. Our aim was to study and correlate the expression of IL-17 and mast cells in lesional and nonlesional scalp areas of patients with FD, using immunohistochemical techniques. There was significantly greater expression of IL-17 and mast cells in lesional compared with nonlesional scalp areas (P <
0.05). We also found a significantly positive correlation between IL-17 and tryptase-immunolabelled mast cells in nonlesional scalp (r = 0.64), highlighting the role of IL-17-secreting mast cells in 'normal-appearing' sites of FD. These findings suggest that novel treatments targeting both IL-17 and mast cells may be potentially beneficial in controlling the chronic inflammatory status of FD, especially for patients with recalcitrant FD.