Klinefelter syndrome (KS) is the most prevalent sex chromosome disorder in man and is a common cause of hypogonadism. To describe mortality in KS, we conducted an epidemiological study, using Danish registers covering the entire nation. We constructed a cohort of 781 Danish boys and men diagnosed with KS (from the Danish Cytogenetic Central Register) and a control group of 3803 men, matched by month and year of birth. Vital status was obtained from the Centralized Civil Register, and causes of death were obtained from the National Register of Causes of Death. We used Cox regression with stratification on groups of diagnoses according to International Classification of Diseases, 10th version. Where significant results were found, subsequent analyses were performed on subdivisions of diagnoses. We found that Klinefelter syndrome was associated with a significant increase in mortality risk of 40% (hazard ratio, 1.40; 95% confidence interval, 1.13-1.74), corresponding to a significantly reduced median survival of 2.1 yr. The increased mortality was mainly due to increased mortality from infectious, neurological, circulatory, pulmonary, and urinary tract diseases. Whether this increase is caused by the syndrome per se (i.e. hypogonadism) or other factors, e.g. socioeconomic, are involved is presently unknown.