In a prospective study, 2,437 U.S. veterans with infectious mononucleosis (IM) diagnosed during military service in World War II were compared with a similar number of controls as to the frequencies of death (1946–65) from leukemia-lymphoma, other cancers, or other causes. No significant differences were observed. There were 5 deaths from lymphoma versus 4 in the controls and versus 2.31 expected on the basis of U.S. national mortality rates. The comparison of 5 deaths observed versus 2.31 expected yielded a relative risk of 2.2 for which the 90% upper and lower confidence limits were 1.1 and 4.0. Retrospective study of 2,659 World War II veterans who died of cancer, 1950–54 or 1959–63, and an equal number of matched controls also revealed no significant difference in the frequency with which IM was diagnosed during military service. Thus neither study demonstrated a relationship between clinical IM and cancer in adult males.