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Notes: Summary Background Ultraviolet (UV) exposure of human skin induces local and systemic immune suppression. This phenomenon has been well documented when UVB radiation (290–320 nm) is used. The mechanism is thought to involve Langerhans cells (LCs), the epidermal dendritic cells that play a crucial role in antigen presentation. A variety of studies have clearly demonstrated that UVB radiation decreases LC density and alters their morphology and immunological function, but little is known about the effects of the entire UV spectrum (ultraviolet solar simulated radiation, UV-SSR or UVB + UVA) or UVA (320–400 nm) radiation alone. Objectives The purpose of this study was to analyse and compare the effects of a single exposure of human volunteers to UV-SSR, total UVA or UVA1 (340–400 nm) in the human epidermal LC density and morphology. Methods Immunohistochemistry on epidermal sheets with various antibodies and transmission electron microscopy (TEM) were used. Results Immunostaining for class II antigen revealed that a single UV-SSR exposure, corresponding to twice the minimal erythemal dose (MED), induced a significant reduction in LC density with only slight morphological alterations of remaining cells. After a single UVA exposure, LC density showed a dose-dependent reduction with a significant effect at 60 J cm−2 (well above the MED). Moreover, the reduction of LC dendricity was also dose-dependent and significant for doses exceeding 30 J cm−2. UVA1 radiation was as effective as total UVA for the later endpoint. As demonstrated by TEM, the location of Birbeck granules containing epidermal cells was modified in UVA-exposed areas. They were located in the spinous rather than in the suprabasal layer. In addition, the morphology of these cells was altered. We observed a rounding up of the cell body with a reduction of dendricity. Alterations of mitochondrial membrane and ridges were also seen. Conclusions A single exposure of human skin in vivo to UV-SSR, UVA or UVA1 radiation results in different alterations of density and/or morphology of LCs. All these alterations may impair the antigen-presenting function of LCs leading to an alteration of immune response.