Greenland Eskimos have one of the highest incidences of nasopharyngeal carcinoma (NPC) in the world. Asian and African areas endemic for NPC show early infection with Epstein-Barr virus (EBV), and few cases of infectious mononucleosis (IM). Among 40000 Eskimos and 10000 Caucasians in Greenland there were 82 reported cases of IM from 1963 to 1980. Incidence rates from 1975 to 1980, age adjusted to the "world" population distribution, were 12.0 and 5.2/100 000/yr for males and females, respectively. Comparable Danish data revealed incidence rates of 97.4 and 105.1. From 1970 to 1981 Greenland hospital records verified 16 cases, of which only 2 were in Eskimos. The IM incidence in Eskimos is extremely low, and Danes living in Greenland have a comparatively much higher incidence.