Abnormalities in several signaling pathways and in the expression and/or activation of different transcription factors in psoriatic keratinocytes have been hypothesized to play a role in the pathophysiology of psoriasis. The mitogen-activated protein kinase (MAPK) cascades are among the best characterized of intracellular signaling pathways, and they play important roles in cell proliferation, differentiation, gene expression, and inflammation. We investigated the expression, activation and distribution of extracellular signal-regulated kinases (ERKs), p38 mitogen-activated protein kinases (p38 MAPK) and c-Jun N-terminal kinases (JNKs), using immunohistochemistry and Western blot in lesional psoriatic skin and normal control skin, to clarify the possible roles of these kinases involved in the pathogenesis of psoriasis. The immunoblot analysis demonstrated that activation of ERK1/2 and p38 MAPK increased in the lesional psoriatic skin. In addition, a significant increase in p-MEK (the upstream activator of ERK), and p-CREB (a downstream transcription factor of active ERK) was also found in our experiment. The immunohistochemical study showed that the levels of phosphorylated ERK1/2 and p38 MAPK were enhanced in lesional psoriatic skin compared with controls. Phosphorylated ERK1/2 and p38 exhibited clear nuclear localization throughout the epidermal part of lesional psoriatic skin. These findings suggested that ERK1/2 and p38 pathways were involved in the pathophysiology of psoriasis.