The clonality of a total of 70 human nasopharyngeal carcinomas (NPC) was analyzed using the structure of the terminal fragment of episomal Epstein-Barr virus (EBV). Thirty female samples heterozygous for the BstXI polymorphism of the phosphoglycerokinase (PGK) gene were analyzed using polymerase chain reaction (PCR) amplification of X-chromosome linked PGK gene for restriction fragment length polymorphism (RFLP). All NPC samples analyzed were shown to be monoclonal, with two exceptions that were polyclonal. Clonal determination was also performed for non-cancerous cell populations: normal, and simple hyperplastic, grade I (mild) and grade II-III (severe) atypical hyperplastic epithelia. It was found that the normal and simple hyperplastic and 3 grade I (mild) atypical hyperplastic epithelia were polyclonal, whereas the grade II-III (severe) atypical hyperplastic samples were monoclonal. The analysis of the clonality of various stages in the neoplastic process suggested that NPC might originate from several cells, after clonal selection; finally a large majority of NPC has been demonstrated to be monoclonal, also indicating that the alteration of clonal nature might have occurred at a very early stage.