Histochemical stainings of frozen sections of human normal and psoriatic skin were used to study the localization of hyaluronan (HA) and intercellular adhesion molecule 1 (ICAM-1). HA staining was found in all areas of the skin, with the exception of the stratum corneum, in both normal and psoriatic cases without any apparent quantitative differences between the conditions. The staining for ICAM-1 was detected in vessels in normal skin and at lower levels in normal areas of the skin in patients with psoriasis. However, in these patients the staining increased to about the same level as in normal skin after hyaluronidase treatment of the sections prior to staining. In psoriatic lesions, distinct staining for ICAM-1 was localized mainly to vessels and infiltrating leukocytes. Treatment of the sections with hyaluronidase increased the staining of vessels only slightly, but more strongly around leukocytes. These findings show that ICAM-1 is predominantly free from bound HA on vessel endothelium in psoriasis lesions but not on vessels in normal areas of the skin, and suggests that systematically administered HA, previously shown to reduce chronic inflammation in animal models, might have a beneficial effect in psoriasis via blocking of endothelial ICAM-1 and thereby causing a reduced invasion of leukocytes into the skin.