BACKGROUND The skin is a dynamic body organ that can be activated by both central and local hypothalamic-pituitary-adrenal axis systems. This phenomenon might be the crucial explanation why stress can cause relapse of chronic inflammatory skin diseases, such as psoriasis. Here, we determined the effects of mast cells on keratinocyte proliferation under stress hormone stimulation. METHODS We subcutaneously injected dexamethasone on the shaved back of mice and evaluated histological changes and keratinocyte growth factor (KGF) expression on dermal mast cells. Further, human mast cell line (HMC-1) and keratinocyte cell line (HaCaT) cells were treated with dexamethasone in vitro to observe the extent of proliferation and the expression of KGF. Finally, the supernatants of HMC-1 cells treated with dexamethasone were used for the culture of HaCaT cells to investigate the effect on proliferation. RESULTS We observed epidermal thickening in dexamethasone-injected mice, accompanied by an increase in the number of KGF-expressing dermal mast cells. Similar to mouse dermal mast cells, KGF was highly expressed in the human mast cell line HMC-1 following stimulation with dexamethasone. Further, dexamethasone-treated mast cells promoted keratinocyte proliferation in vitro. However, the effects of mast cells on keratinocytes were significantly diminished in the presence of anti-KGF-blocking antibodies. CONCLUSIONS Taken together, our results show that a stressful environment may disturb skin barrier homeostasis through mast cell-derived KGF expression.