The risk of solid and hematological malignancy in patients with Turner syndrome, characterized by X chromosome monosomy in women, and Klinefelter syndrome, characterized with two and more X chromosomes in men, is not well established, but such evidence may have etiological implications on cancer development. We identified a total of 1,409 women with Turner syndrome and 1,085 men with Klinefelter syndrome from the Swedish Hospital Discharge and Outpatient Register. These individuals were further linked to the Swedish Cancer Register to examine the standardized incidence ratios (SIRs) of cancer using the general population without Turner and Klinefelter syndromes as reference. The overall risk of cancer was 1.34 for women with Turner syndrome; it was increased only for solid tumors. For a specific type of tumor, the risk of melanoma and central nervous system tumor was significantly increased. For persons with Klinefelter syndrome, the risk of solid tumors was decreased (SIR = 0.66), whereas the risk of hematological malignancy was increased (SIR = 2.72). Non‐Hodgkin lymphoma and leukemia showed an increased SIR of 3.02 and 3.62, respectively. Our study supported the hypothesis that X chromosome plays an important role in the etiology of solid tumors. The underlying mechanisms for the increased incidence of non‐Hodgkin lymphoma and leukemia in persons with Klinefelter syndrome need to be investigated further.