In addition to hyperproliferation of keratinocytes, psoriasis is characterized by pronounced leukocytic infiltration. In contrast to the epidermal localization of neutrophils and T lymphocytes, macrophages are almost exclusively restricted to the dermal compartment. By immunohistologic analysis, these dermal macrophages were mainly encountered in the papillary dermis and arranged along the rete ridges in close proximity to proliferating keratinocytes. Monocyte chemoattractant protein (MCP-1) anti-sense RNA probes yielded abundant signals over the proliferating basal keratinocytes of the tips of the rete ridges, and, to a lesser extent, in cells in the papillae. Thus, the strongest MCP-1 message in psoriatic lesions is found above the dermal-epidermal junction and this may explain the characteristic sub-basal distribution of dermal macrophages. These results suggest that MCP-1 is important in regulating the interaction between proliferating keratinocytes and dermal macrophages in psoriasis pathogenesis.