Kaposi's sarcoma (KS) developed in a patient with human T-cell leukemia virus type I (HTLV-I)-associated adult T-cell leukemia who was treated with a short-term course of monoclonal antibody immunotherapy. The presentation was transient and temporally related to the underlying clinical course. The association of KS in an HTLV-I infected, but not human immunodeficiency virus (HIV)-infected, individual should alert investigators to the occurrence of KS in retroviral-associated diseases other than acquired immunodeficiency disease syndrome. Recognition of the similarities and differences between HTLV-I and HIV infections may provide insights concerning the angiopathogenesis of KS.