Chronic actinic dermatitis (CAD) is a photosensitivity disorder marked by severe eczematous lesions on exposed areas. Although associations with contact dermatitis, atopic dermatitis, and human immunodeficiency virus (HIV) have been suggested, its pathogenesis remains unknown. CAD is often refractory, and systemic administration of cyclosporin A has been the treatment of choice. Recently, topical tacrolimus therapy has been reported to be effective. We report the efficacy of topical tacrolimus treatment in a CAD patient who also had the complication of idiopathic leukopenia. A phototest showed marked suppression of erythema formation in the skin pre-treated with tacrolimus before UVB radiation but not in the skin treated after the irradiation. Therefore, it is suggested that tacrolimus may prevent UV-B induced erythema by suppressing a very early phase of the inflammatory process in CAD.