Psoriasis is a proliferative and inflammatory disease of the skin. Caspase 9 is responsible for initiating the caspase activation cascade during apoptosis. Apoptosis is a physiological mechanism of homeostasis and development, and caspases are the executioners of apoptosis. This study reports the immunohistochemical localisation of caspase 9 in psoriatic skin and compares it with that seen in normal, healthy control skin. Skin biopsy specimens of lesions were obtained from 15 patients with plaque type psoriasis vulgaris. The specimens were labelled immunohistochemically for binding of an anti-caspase 9 primary antibody. Biopsies of healthy skin from 10 age-matched and sex-matched healthy control individuals were also analysed. The caspase 9 positive cell fraction was calculated for both epidermal and dermal cells in psoriatic lesions and healthy control skin. Counts of caspase 9 positive cells from the epidermis of psoriatic skin lesions were significantly lower than those seen in healthy skin (p < 0.05). The caspase 9 immunolabelled perivascular cell counts in the dermis were not statistically significantly different in psoriatic lesions versus normal skin (p > 0.05). Psoriatic epidermis contains little of the apoptotic marker, caspase 9. The results of this study are indicative of decreased apoptosis in psoriatic epidermis, and no change in the perivascular area in psoriatic lesions. These findings support the idea that decreased apoptosis is seen in psoriatic epidermal cells. Greater understanding of the nature of the disease may open new avenues for further therapeutic modalities. (C) 2006 Elsevier GmbH. All rights reserved.