Kaposi's sarcoma (KS) occurs in Europe and the Mediterranean countries (classic KS) and Africa (endemic KS), immunosuppressed patients (iatrogenic or post–transplant KS) and those with acquired immunodeficiency syndrome (AIDS), especially among those who acquired human immunodeficiency virus sexually (AIDS–KS). KS–associated herpesvirus (KSHV or HHV–8) is unusual among herpesviruses in having a restricted geographical distribution. Like KS, which it induces in immunosuppressed or elderly people, the virus is prevalent in Africa, in Mediterranean countries, among Jews and Arabs and certain Amerindians. Distinct KSHV genotypes occur in different parts of the world, but have not been identified as having a differential pathogenesis. KSHV is aetiologically linked to three distinct neoplasms: (i) KS, (ii) primary effusion lymphoma, and (iii) plasmablastic multicentric Castleman's disease. The histogenesis, clonality and pathology of the tumours are described, together with the epidemiology and possible modes of transmission of the virus.