T-cell infiltration in plaque psoriasis has recently been an important subject of investigation. Interestingly, comparative analyses of the disease-specific composition of the lesional T-cell infiltrate in plaque psoriasis and other inflammatory dermatoses have only sparsely been performed. To compare plaque psoriasis vs. atopic dermatitis and lichen ruber planus with respect to T-cell subsets, epidermal proliferation and keratinization. Biopsies were taken from untreated lesional skin of patients, six with psoriasis, six with atopic dermatitis and six with lichen planus. T-cell subsets (CD4+, CD8+, CD45RO+, CD45RA+, CD2+, CD25+), an epidermal proliferation (Ki-67) and a keratinization marker (K10) were stained immunohistochemically and quantified using image analysis. The high number of CD8+ T cells (52 +/- 13 cells mm(-1)) found in the psoriatic epidermis was not found in the epidermis of atopic dermatitis (9 +/- 4), nor in the epidermis of lichen planus (34 +/- 10). The other T-cell subsets in the epidermis and dermis showed no statistically significant differences between psoriasis and atopic dermatitis. In contrast to the limited presence of CD4+, CD8+ and CD2+ in the psoriatic dermis (110 +/- 19, 27 +/- 9, 127 +/- 41, cells mm(-1), respectively), more impressive numbers of these cells were observed in the dermis of lichen planus (300 +/- 53, 144 +/- 38, 272 +/- 48, respectively). CD45RO+ memory effector T-cell counts were significantly higher in the epidermis of lichen planus (39 +/- 10) than in psoriasis (19 +/- 5). Psoriatic epidermis proved to have major keratinocyte hyperproliferation (247 +/- 26 cells mm(-1) lamina basalis), as compared with atopic dermatitis (134 +/- 15) and lichen planus (128 +/- 20). Furthermore, a marked decreased expression of keratin 10 was observed in psoriasis (41% of epidermal area) contrary to atopic dermatitis (70%). Psoriatic epidermis exhibits a pronounced CD8+ epidermotropism with accompanying epidermal hyperproliferation and abnormal keratinization, which changes are only minimally expressed in atopic dermatitis and lichen planus. In plaque psoriasis, substantially fewer activated CD4+ and CD8+ T cells in the dermis and less CD45RO+ T cells in the epidermis are present in comparison with lichen ruber planus.