HIV protease inhibitors have been shown to exert antiangiogenic and antitumor actions independently from their antiretroviral effect. Based on these studies, HIV-seronegative patients with classic Kaposi's sarcoma were treated with indinavir and followed for clinical evolution, drug pharmacokinetics and Kaposi's sarcoma biomarkers. A favorable clinical course was associated with high drug plasma levels, reduced production of basic fibroblast growth factor, lower numbers of circulating endothelial cells, and a decrease in antibody titers against human herpesvirus 8.