The only well-established risk factors for childhood leukemia are high-dose ionizing radiation and Down syndrome. Computerised tomography is a common source of low-dose radiation. In this study, we examined the magnitude of the risk of childhood leukemia after pediatric computed tomography examinations. We evaluated the association of computed tomography scans with risk of childhood leukemia in a nationwide register-based case-control study. Cases (N=1093) were identified from the population-based Finnish Cancer Registry and three controls matched by gender and age were randomly selected for each case from the Population Registry. Information was obtained also on birth weight, maternal smoking, parental socioeconomic status and background gamma radiation. Data on computed tomography scans were collected from the ten largest hospitals in Finland, covering approximately 87% of all pediatric computed tomography scans. Red bone marrow doses were estimated with NCICT dose calculation software. The data were analyzed using exact conditional logistic regression analysis. A total of 15 cases (1.4%) and 10 controls (0.3%) had undergone one or more computed tomography scans, excluding a two-year latency period. For one or more computed tomography scans, we observed an odds ratio 2.82 (95% CI 1.05, 7.56). Cumulative red bone marrow dose from computed tomography scans showed an excess odds ratio 0.13 (95% CI 0.02, 0.26) per mGy. Our results are consistent with the notion that even low doses of ionizing radiation increase observably the risk of childhood leukemia. However, the observed risk estimates are somewhat higher than those in earlier studies probably due to random error, but unknown predisposing factors cannot be ruled out.