KS and non-Hodgkin's lymphomas frequently involve the gut in patients with AIDS. These neoplasms establish the diagnosis of AIDS in an HIV-positive patient. KS is a spindle-cell tumour derived from lymphatic endothelia which is associated with luminal lesions in at least 40% of patients. Gastrointestinal KS is usually asymptomatic but may rarely bleed or obstruct. Treatment of KS with either interferon-α, radiation or chemotherapy can reduce tumour bulk, but does not alter overall survival in AIDS. Non-Hodgkin's lymphomas in AIDS are B cell neoplasms with many genotypic and phenotypic similarities to Burkitt's lymphoma. The tumours are usually highly aggressive, and present in extranodal sites in the majority of cases. Of these extranodal sites, gastrointestinal involvement is most common. Gastrointestinal lymphomas are usually symptomatic and almost always require treatment. Obstruction, perforation and bleeding may occur in patients with luminal involvement, whereas hepatic or biliary disease may lead to jaundice. Several chemotherapeutic regimens for lymphoma have been successfully used to achieve partial remission, although no prolongation of survival has been demonstrated. There appears to be an increased incidence of Hodgkin's disease in patients with AIDS, which is generally of advanced stage. This tumour does not meet the CDC criteria for AIDS as yet. Hepatic and/or splenic involvement in this setting are common.