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Background: Men with Klinefelter syndrome have one or more extra X chromosomes and have endocrine abnormalities. Case reports have led to suggestions that men with Klinefelter syndrome have elevated risks of several cancers, but published cohort studies have been relatively small. We conducted a nationwide cohort study to examine these risks. Methods: We followed a cohort of 3518 men who had been cytogenetically diagnosed with Klinefelter syndrome in Britain from 1959 through 2002 and compared their cancer incidence and mortality with that of men in the national population. All statistical tests were two-sided. Results: The standardized mortality ratio (SMR) for all cancers was 1.2 (95% confidence interval [CI] = 1.0 to 1.4). Compared with the general population, men with Klinefelter syndrome had higher mortality from lung cancer (SMR = 1.5, 95% CI = 1.0 to 2.0), breast cancer (SMR = 57.8, 95% CI = 18.8 to 135.0), and non-Hodgkin lymphoma (SMR = 3.5, 95% CI = 1.6 to 6.6) and lower mortality from prostate cancer (SMR = 0, 95% CI = 0 to 0.7). The standardized mortality ratios were particularly high for breast cancer among men with 47,XXY mosaicism (SMR = 222.8, 95% CI = 45.9 to 651.0) and for non-Hodgkin lymphoma among men with a 48,XXYY constitution (SMR = 36.7, 95% CI = 4.4 to 132.5). The cancer incidence data corroborated these associations. Conclusions: These results support a hormonal etiology for breast cancer in men and for prostate cancer and suggest that men with Klinefelter syndrome may be at substantially elevated risks for non-Hodgkin lymphoma, breast cancer, and, perhaps, lung cancer. [ABSTRACT FROM AUTHOR]