Heparin-binding EGF-like growth factor (HB-EGF) and amphiregulin (AREG) are the members of EGF family that bind to common EGF receptor (EGFR) in the epidermis. However, the role of these two growth factors in epidermal hyperplasia of psoriasis has not been established. On the other hand, CD4+ T cells are responsible for the development of the psoriatic plaques. However, inflammatory cytokines, such as TNFalpha, IL-1beta and IFNgamma, inhibit the growth of human keratinocytes in vitro. The expression of HB-EGF, AREG and EGFR proteins in normal (n = 22) and psoriatic (n = 34) skin tissues was examined by immunohistochemistry. Then, the effects of HB-EGF and AREG on the growth of cultured adult normal human epidermal keratinocytes (NHEK-AD) with or without TH1 cytokines, such as TNFalpha, IL-1beta and IFNgamma, were examined by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay, and the effects of these cytokines on the expression of EGFR mRNA in NHEK-AD were examined by real-time reverse transcriptase-polymerase chain reaction. The expression of HB-EGF and AREG in the epidermis was not specific to psoriatic plaques, but the distribution of positive cells throughout the epidermis was different between normal skins and psoriatic plaques. On the other hand, in the dermis and the papillary dermis, most of vascular endothelial cells and infiltrating mononuclear cells expressed both HB-EGF and AREG in normal skins and psoriatic plaques, and these positive cells were more frequent in psoriasis compared to normal skin. In the in vitro growth assay, HB-EGF, not AREG, stimulated the proliferation of NHEK-AD at the optimal concentration of 1 ng/ml. Furthermore, HB-EGF compensated the growth-suppressing effects of TNFalpha, IL-1beta and IFNgamma on NHEK-AD, and TNFalpha promoted the growth of NHEK-AD at the concentration of 2 and 20 U/ml in combination with HB-EGF and, in lesser extent, with AREG. However, TNFalpha did not affect the expression of EGFR mRNA in NHEK-AD. Growth factors and inflammatory cytokines produced in the dermis would be important for the epidermal proliferation in psoriatic plaques and TNFalpha may play a key role in cooperation with HB-EGF and AREG in the proliferation of epidermal keratinocytes at the psoriatic skin lesions.