- BACKGROUND: Acitretin has been shown to be effective for psoriasis treatment. Its mechanism of action is not completely understood, and there are few studies focusing on histological and immunohistochemical differences before and after treatment of psoriasis with acitretin. METHODS: This is a prospective study of 17 patients with plaque psoriasis treated with acitretin for 4 months with biopsies taken before and after therapy. Histological features and immunohistochemical reactions to cytokeratin (CK) 10, CK16, CK19, Ki67 and CD1a were evaluated and compared. RESULTS: There were nine men and eight women with median age of 47 years. Epidermal thickness, CK16 positivity, Ki67 and CD1a-positive cell index reduced after treatment (p < 0.01). Suprapapillary plate thickness stayed the same (p > 0.05) although the epidermal/suprapapillary thickness ratio was significantly higher before treatment (p < 0.01). CK10 positivity was lower and a thicker basal cell layer was seen in the epidermis before treatment (p < 0.01). CK19 was negative in all cases. CONCLUSIONS: Acitretin therapy improved histological and immunohistochemical features typical of psoriasis. In psoriasis, suprapapillary plates are not thin, but the epidermal/suprapapillary thickness ratio is increased. Basal cell layer is expanded in psoriasis. Langerhans' cells were less frequent after treatment, and that finding has to be investigated further to determine its role in acitretin mechanism of action.