In order to establish if immunologic alterations seen in acquired immunodeficiency syndrome (AIDS)-associated Kaposi's sarcoma are also present in the primary form of this malignancy, peripheral blood T-lymphocyte characterization and HLA A, B, C, and DR phenotyping have been carried out in 18 patients with the primary form of the disease. HLA antigen distribution was not significantly different from that of the normal population and, in particular, DR5 was only marginally increased. The T4/T8 ratio was significantly higher in the patients than in the control group, and this was due to a marked decrease in suppressor cells. The possible pathogenetic relevance of these findings is discussed.