OBJECTIVE The effect of the established antipsoriatic treatment with topical calcipotriol (with a maximum of 100 g per week) in addition to systemic treatment with alefacept, a new biological agent for psoriasis, on epidermal cell populations in the psoriatic lesion was investigated using a combination of the Zenon labelling technique and microscopic image analysis. Epidermal cell populations were measured quantitatively with this sensitive method. METHODS Frozen sections of non-treated psoriatic epidermis and psoriatic epidermis treated with either alefacept intramuscular or alefacept intramuscular in combination with topical calcipotriol for 12 weeks were compared immunohistochemically. Antibodies against keratin 6, 10 and 15 were labelled with the Zenon technique, whereas antibodies against the Ki-67 antigen and beta-1 integrin were covalently Fluorescein Isothiocyanate (FITC)-labelled. Using image analysis, these markers were measured in the epidermis in a standardized manner. CONCLUSIONS Treatment of psoriasis with alefacept resulted in a good clinical response in several patients and in a normalization of epidermal expression of the immunohistochemical parameters for differentiation and proliferation. The addition of topical calcipotriol resulted in a faster clinical improvement with a similar overall clinical response and a similar response of epidermal cell populations as compared to treatment with alefacept monotherapy after 12 weeks of treatment. This study also suggests that the appearance of keratin 15 has a predictive value for the duration of remission. It can be concluded that the addition of a low-dose calcipotriol treatment does not contribute to the clinical efficacy of alefacept, both at the clinical level and with respect to markers for epidermal proliferation and differentiation.