Kaposi's sarcoma (KS) is a frequent cause of morbidity and mortality in patients with human immunodeficiency virus (HIV) infection. Several characteristics of KS pose challenges for the conduct of clinical trials. Kaposi's sarcoma patients often have multiple, irregularly shaped lesions, making accurate assessment of tumor size difficult. The lesions may have varying degrees of nodularity. Involvement of the lung or other visceral organs often consists of multiple irregular lesions. Conventional oncology staging systems cannot be applied effectively to KS because there is no clear primary lesion. Kaposi's sarcoma is affected by the status of the underlying HIV infection, and there are reports of KS lesions regressing in response to effective antiretroviral therapy. A system for staging and response assessment in KS, developed by the AIDS Clinical Trials Group (ACTG), has proven to be a useful tool for the conduct of trials in KS. A newer system that also attempts to assess patient benefit in response to therapy is now being developed by the National Cancer Institute, FDA, and AIDS Malignancy Consortium. These tools, as well as careful methodology in the conduct of clinical trials, should help optimize the clinical development and evaluation of new therapies for KS.