BACKGROUND: Early studies using HIV protease inhibitors (PI) showed regression of Kaposi's sarcoma (KS) lesions in some patients. OBJECTIVE: Our purpose was to determine prospectively the influence of PI on HIV-related KS. METHODS: KS lesions of nine patients with progressive cutaneous disease were prospectively evaluated clinically and by means of epiluminescence microscopy before and during PI therapy. HIV viremia and CD4 cell count were measured in parallel. RESULTS: All patients experienced reduction or initial stabilization of KS lesions during the first 4 to 8 weeks of HIV-1 PI therapy. After a median follow-up of 7 months and according to AIDS Clinical Trials Groups criteria, six patients had a partial response, two showed stable disease, and in one noncompliant patient KS progressed, requiring chemotherapy. With epiluminescence microscopy, a reduction in skin surface alterations, lesional size, and color intensity was demonstrated in six of nine patients. PI induced a median decrease in viremia of 1.66 log and a median increase in the CD4 count of 49 cells/mm3. CONCLUSION: In this series, HIV PI therapy reduced or stabilized KS. The efficacy of HIV-1 PI in KS may result from the improvement in cellular immunity. These results suggest the use of PI in AIDS-related KS regardless of the level of CD4 lymphocyte count and HIV viremia.