Adult-onset Still disease (AOSD) is an uncommon disorder characterized by fever, polyarthralgia, elevated white blood cell count, and a maculopapular rash, the histologic features of which have not been well-known. A 55-year-old Asian woman presented initially with a ''burning'' and severely pruritic eruption on her face, hands, and arms, thought clinically to be urticaria. Within 1 month, she began spiking high fevers, developed diffuse joint pain, and had marked elevations of ferritin, C-reactive protein, and erythrocyte sedimentation rate, characteristic of AOSD. The cutaneous eruption became more widespread, involving the trunk, scalp, and remainder of the extremities, with diffuse thickening of the skin with papular and linear hyperpigmentation and accentuation. Biopsies from several locations showed focal hyperkeratosis associated with dyskeratotic keratinocytes with a peculiar, distinctive distribution in the upper epidermis and cornified layers. In addition, increased dermal mucin was present, with minimal fibroblast proliferation and inflammation. This unusual combination of diffuse dermal mucinosis and a unique pattern of dyskeratosis can present a challenge in generating an accurate differential diagnosis, and may represent an unusual response to chronic scratching or be a distinctive histologic manifestation of AOSD. (Am J Dermatopathol 2007;49:194-196)

The development of juvenile xanthogranuloma (JXG) as a sequel to langerhans cell histiocytosis (LCH) treated with chemotherapy is rare and the hypothesis is intriguing. This is a case of a 19-year-old woman who presented with progressive development of tan-red papules on the axilla and eyelids over a 1.5-year time span. A biopsy of an axillary lesion showed a prominent dermal infiltrate of foamy histiocytoid cells with occasional Touton-type multinucleate giant cells, consistent with JXG. Three years later, the patient presented with additional similar papules on the axilla and vulva as well as a painful mass in the pelvic bone and diabetes insipidus with an associated pituitary mass. An iliac crest bone biopsy showed an eosinophil-rich infiltrate admixed with histiocytoid cells with reniform nuclei, which expressed S100 and CD1a, consistent with a diagnosis of LCH. Nonetheless, an additional axillary papule was once again consistent with JXG, with negative reaction for S100 and CD1a with no Birbeck granules by electron microscopy. This case is unique by the co-existing presentation of multiple cutaneous JXG lesions and internally confined LCH lesions without an apparently associated chemotherapy, corroborating the concept that JXG and LCH may share a common histogenesis.