Poliomyelitis has hypothetically been associated with an increased risk of central nervous system (CNS) tumors. The present study was performed to examine not only the risk of CNS tumors but also the overall risk of cancer among a cohort of 5,883 polio patients. Patients diagnosed with acute poliomyelitis in the Danish capital, Copenhagen, between 1919 and 1954 were identified and followed with respect to cancer. Information on vital status and cancer diagnoses was obtained through linkage with the Danish Civil Registration System and the Danish Cancer Registry, respectively. The ratio of observed number of cancers to the number expected from population-based incidence rates, i.e., the standardized incidence ratio (SIR), served as measure of the relative cancer risk. Overall, 717 cases of cancer were observed among 5,883 polio patients during 249,084 person-years of follow-up vs. an expected number of 645 (SIR β«Ψβ¬ 1.11 [95% confidence interval 1.03 to 1.20])). The increased risk was restricted to female polio patients (SIR β«Ψβ¬ 1.18 [1.07 to 1.30]), among whom the risk was particularly high for breast cancer (SIR β«Ψβ¬ 1.35 [1.12 to 1.61]) and for skin cancer (SIR β«Ψβ¬ 1.66 [1.32 to 2.07]). The risk of breast cancer was highest among women with a history of paralytic polio (SIR β«Ψβ¬ 1.62 [1.24 to 2.10]). The observed number of CNS tumors did not exceed the expected (SIR β«Ψβ¬ 1.09 [0.72 to 1.60]). Women diagnosed with poliomyelitis, in particular paralytic polio, may be at increased risk of breast cancer. There was no association between malignancies of the CNS and poliomyelitis.