By virtue of its capacity to serve as a counter-receptor for lymphocyte function-associated antigen-1, intercellular adhesion molecule-1 (ICAM-1) plays a pivotal role in generation and maintenance of immunologic/ inflammatory skin diseases by mediating leukocyte/ keratinocyte adhesion. Ultraviolet radiation (UVR) may exert both antiinflammatory effects (e.g., UV phototherapy) and proinflammatory effects (e.g., triggering of photosensitive skin diseases) on human skin. Recent evidence indicates that UVR-induced changes of keratinocyte ICAM-1 expression constitute the molecular basis for these ambivalent properties of UVR, as UVR is able to exert two separate and even opposite effects on ICAM-1 expression. As an antiinflammatory effect, UVR may inhibit cytokine-induced up-regulation of keratinocyte ICAM-1 expression, whereas induction of ICAM-1 expression by UVR represents a proinflammatory activity. This latter effect is mediated by an autocrine mechanism involving interleukin (IL)-lα. In this autocrine system, UVR exposure of human keratinocytes leads to the release of IL-1α, which in turn up-regulates the expression of IL-1 receptor type 1 molecules on the keratinocyte surface, thereby increasing the sensitivity of these cells toward IL-lα. As a consequence, irradiated keratinocytes are capable of responding to endogenously produced IL-lα by increasing ICAM-1 expression. Modulation of keratinocyte ICAM-1 expression after UVR exposure may be observed after both short-wave UVR (UVB; 280–320 nm) and longwave UVR (UVA1; 340–400 nm). The photobiologic mechanisms underlying UVB versus UVA1 radiation-induced ICAM-1 modulation have been found to differ. Although not completely delineated, UVB radiation-induced modulation of ICAM-1 expression appears to be mediated via the induction of DNA damage, whereas UVA1 radiation effects involve the generation of reactive oxygen intermediates. J Invest Dermatol 105:67S-70S, 1995