Cancer and birth defects may share factors that influence risk. A malformation may involve physiologic changes or changes in lifestyle that might affect cancer risks. In Norway and Sweden, the population-based medical birth and cancer registries were linked to identify subsequent cancer occurrence in children with birth defects and among their parents and siblings. Altogether, 5.2 million children and their families were included. The standardized incidence ratio (SIR) served as a measure of relative risk. There was an increased overall cancer risk in individuals with birth defects in the two countries [SIR, 1.7; 95% confidence interval (95% CI), 1.6-1.9], and the increased risk remained into early adulthood. Individuals with malformations in the nervous system were at increased risk of developing cancer in the brain/nervous system (Norway: SIR, 58; 95% CI, 41-80; Sweden: SIR, 8.3; 95% CI, 4.0-15), individuals with Down syndrome were at an increased risk of leukemia (Norway: SIR, 36; 95% CI, 26-48; Sweden: SIR, 36; 95% CI, 28-46), and there was an increased overall cancer risk for individuals with multiple birth defects (Norway: SIR, 5.5; 95% CI, 3.3-8.7; Sweden: SIR, 3.6; 95% CI, 2.2-5.4). There was no increased overall cancer risk among mothers (SIR, 1.0; 95% CI, 1.0-1.0), fathers (SIR, 1.0; 95% CI, 0.9-1.0), and siblings (SIR, 1.0; 95% CI, 0.9-1.1) of children with birth defects. We observed an increased overall cancer risk in individuals with birth defects. The highest risks were seen for individuals with malformations in the nervous system, Down syndrome, and multiple defects. No increased overall cancer risk was seen among their parents or siblings.