The epidermal phenotype as observed in psoriatic skin results from inflammation and abnormal proliferation and terminal differentiation of keratinocytes. Mice deficient for interferon regulatory factor-2, a repressor of interferon signaling, display psoriasis-like skin inflammation. The development of this phenotype is strictly dependent on type I interferon (interferon-alpha/beta) signaling. The aim of this study was to assess the involvement of interferon-alpha/beta in the pathogenesis of human psoriasis. In psoriatic skin, we measured an increased expression of components that play central and crucial roles in interferon-alpha/beta signal transduction. Culturing keratinocytes or healthy skin biopsies with recombinant interferon-alpha stimulated this signaling pathway; however, this did not induce the expression of markers that are generally used to define the psoriasis phenotype. Furthermore, skin from psoriasis patients responded identically to interferon-alpha stimulation, demonstrating that psoriatic skin does not have an aberrant sensitivity to type I interferon. We conclude that in psoriatic lesional skin the type I interferon signaling pathway is activated, despite an unaltered interferon-alpha sensitivity. Our data furthermore show that type I interferon, in contrast to interferon-gamma, does not act directly on keratinocytes to induce a psoriatic phenotype. Thus, if the observed activated type I interferon signaling is indeed functionally involved in the pathogenesis of psoriasis, its contribution might be indirect, putatively involving other cell types besides keratinocytes.