Human herpes virus 6 (HHV 6) has been implicated as the causative agent of exanthema subitum in young children. Recently, we reported two cases of a severe, infectious, mononucleosis-like syndrome resulting from a primary HHV 6 infection in immunocompetent adults. Both of these patients had the skin condition generally referred to as "erythroderma". A skin-biopsy specimen from one of them, a 43 year old man, was examined. Using immunohistochemical staining and in situ hybridisation, lymphocytes infected with HHV 6 were found in the skin. It is proposed that the erythroderma in immunocompetent adults infected with primary HHV 6 is provoked by infiltration of infected inflammatory cells or infected neoplastic lymphocytes into the dermis.