The effect of recombinant tumor necrosis factor-alpha (rTNF), injected directly into the tumor, was evaluated in a Phase I/II study of 27 patients with AIDS-associated Kaposi's sarcoma (KS). The maximally tolerated intralesional dose was less than 100 micrograms/m2 and the recommended intralesional dose was 25 micrograms/m2. In a double-blind, randomized, placebo-controlled study, rTNF reduced the cross-sectional area of 15 of 16 (94%) of the injected KS lesions and caused complete disappearance of 3 of 16 (19%) lesions. Only injected lesions showed a response. Rigors and fever were common dose-dependent side effects and were attenuated by meperidine. There were no changes in human immunodeficiency virus (HIV) activity as determined by serum p24 antigen levels. While biologically active, the systemic toxicity of rTNF as well as the lack of distant antitumor effects in noninjected lesions limits its clinical usefulness under the conditions employed in this trial.