The classic clinical manifestations of Klinefelter syndrome (KS) are expressions of the primary hypogonadism that causes severe alterations of the reproductive and endocrine functions of the testis. It is a syndrome that causes infertility, and in addition leads to multiple disorders that involve a variety of tissues and organs. Important medical conditions associated with KS are categorized as: 1) motor, cognitive, and behavioral dysfunction; 2) tumors; 3) vascular disease and 4) endocrine/ metabolic and autoimmune diseases. The overall incidence of cancer in men with this syndrome is similar to that of the general population, but some malignancies show a significantly higher prevalence in these patients. It is possible that the increased risk of developing certain cancers can be attributed to a direct effect of the chromosomal abnormality (the supernumerary X chromosome), or the combined action of the abnormal chromosomes and hormonal imbalances. Although data in the literature on cancer and KS are abundant, most of them are individual case reports. Only three epidemiological studies with relatively large cohorts provide data with greater reliability, although each has inherent imitations related to study design. This review paper summarizes the current knowledge about cancer risk from childhood to adulthood in patients with KS.