A prospective study of EB virus infections was initiated in July, 1969 in the entering class of 1401 cadets, at the U.S. Military Academy at West Point, N.Y. and continued over 4 yr. On entry 63.5% possessed EBV antibody and 36.5 lacked EBV antibody. The rate of antibody prevalence varied with the geographic area from which the cadet originated.Except in two cadets already ill on first bleeding no evidence of clinical infectious mononucleosis (I.M.) occurred over the 4 yr period in the 890 cadets entering the Academy with EBV antibody. Among 437 cadets without antibody on entry, 54 or 12.4% were infected (seroconverted) in the freshman year; 15 of these had clinical I.M., 12 had suggestive I.M., and 39 had no known mono-like illness. The annual infection rates in susceptible cadets in the second, third, and fourth years were 24.4, 15.1, and 30.8 per 100, respectively. Of 201 cadets infected with EBV over 4 yr only 26.4% were manifested by heterophile positive clinical infectious mononucleosis. Overall, 46% of the 437 cadets entering without EBV antibody became infected over 40 mo of serologic observation; definite clinical infectious mononucleosis developed in 53 cadets, a clinical attack rate of 12.1 per 100 for 4 yr. The EBV infection rate among exposed and susceptible roommates of known cases was no higher than in roommates not so exposed.Elevations of EBV-specific and total IgM occurred during acute illness and disappeared in late convalescence. Total IgG and IgA levels were less commonly elevated. EBV-specific-IgM antibody was demonstrable during the acute illness but was absent 12 mo later. Analysis of EBV infection rates revealed no difference among persons of different ABO blood groups.