Epstein-Barr virus (EBV) causes infectious mononucleosis as a primary disease. The virus infects more than 90% of the average population and persists lifelong in peripheral B-lymphocytes. The virus is produced in the parotid gland and spread via the oral route. Serology suggests that the Epstein-Barr virus might be involved in the causation of two neoplastic diseases of humans: African Burkitt's lymphoma and nasopharyngeal carcinoma. Whereas the development of the lymphoma has an even better linkage with chromosomal rearrangements, nasopharyngeal carcinoma shows a unique association with Epstein-Barr virus. Environmental factors, including traditional Chinese medicine, may be responsible for the enhanced risk of nasopharyngeal carcinoma in certain, predominantly Chinese, populations of southern Asia. Possible mechanisms leading to the establishment of the neoplastic manifestations will be discussed.