This paper describes a case report of an HIV-infected patient with mucocutaneous Kaposi's sarcoma (KS) with oral involvement, which presented complete clinical resolution of lesions on antiretroviral treatment with ritonavir, an HIV-1 protease inhibitor. Although it has still not been demonstrated that ritonavir has a specific antiviral action against HHV-8, a gamma herpesvirus probably involved in KS aetiopathogenesis, it has been proven that it reduces the HIV load significantly. This affects certain growth factors of KS, such as Tat protein and cytokines, and favours recovery of immune function, which correlates with protection against AIDS-defining conditions.