With the exception of malignancy of the testes and leukemia, SMORs for the major classes of malignant neoplasms were significantly lower than expected. For example, the SMORs for liver, uterine, bladder, hematopoietic (excluding leukemia), and pancreas cancer were all less than 0.5. We found an SMOR for testicular cancer of 3.2, a finding consistent with previous studies suggesting that testicular cancer is unusually frequent among men with Down syndrome.