After three months of corticosteroid treatment, a sixty-nine-year-old man, suffering from temporal arteritis, developed a Kaposi's sarcoma (KS) initially located on the left ankle and subsequently spread over both feet and hands. Laboratory data showed a deficiency of both humoral and cellular immunity and constant positivity in the tests for cytomegalovirus. The onset of KS during corticosteroid treatment of temporal arteritis is an extremely rare occurrence, this case being only the second one reported in the literature. In our case the development of this neoplasm can be related to an immunodeficiency that led to a deficit in the immunological surveillance, along with an activation of oncogenic viruses.