A study was undertaken to estimate the magnitude of association between self-reported infectious mononucleosis (IM) and 6 types of cancer, including Hodgkin's disease, non-Hodgkin's lymphoma, nasopharyngeal cancer, nasal cancer, primary liver cancer, and sarcoma. Cases were male, aged 15-39 y in 1968, who lived in 8 cancer registry areas. Controls were men selected by random-digit telephone dialing. Cases included 1511 persons with non-Hodgkin's lymphoma, 343 with Hodgkin's disease, 386 with sarcoma and 168, 113 and 70 with primary liver, nasopharyngeal and nasal cancers, respectively. There were 1910 controls. For the 6 cancers combined, the overall odds ratio for IM occurring < 5 and > or = 5 y of the reference date were 5.40 [95% (Confidence Interval (CI) = 1.61, 18.09] and 1.08 (0.84, 1.40), respectively. Analogous values were 4.59 (1.25, 16.85) and 1.07 (0.78, 1.48) for non-Hodgkin's lymphoma and 7.49 (1.52, 36.92) and 1.35 (0.87, 2.09) for Hodgkin's disease. There was the suggestion of a protective association with IM occurring > or = 5 y before cancer onset for the 4 non-lymphomatous cancers. Strongly positive associations between self-reported IM and 6 types of cancer were observed for IM occurring < 5 y before the onset of cancer. There was a suggestion, which is noted with extreme caution, that IM earlier in life might have had a protective association with the 4 non-lymphomatous cancers.