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This proposal aims to utilize the pandemic as a “natural experiment”, by studying childhood cancer incidence before, during and after the pandemic in Sweden, Denmark, and Finland. The pandemic has provided a unique opportunity to test the hypothesis of an infectious disease etiology for acute lymphoblastic leukemia, enhanced by the different pandemic strategies in the Nordic countries. We also aim to study the impact of the COVID-19 pandemic on childhood cancer patients, both regarding susceptibility to COVID-19 in long-term childhood cancer survivors, and, for those diagnosed with childhood cancer during the pandemic, whether the path through primary care to cancer diagnosis and short-term survival has changed compared to the period prior to the pandemic. An important aspect of this project is that we will study if the social inequalities in number of contacts with healthcare and childhood cancer survival have worsened during the pandemic.We will utilize the unique infrastructure for register-based research in the Nordic countries: for incidence trends and health care pattern around the pandemic, new data will be collected from national health data registers and regional primary care. For COVID-19 risk in long-term survivors the unique Nordic cohort of childhood cancer survivors, SALiCCS, will be utilized, including 20 000 survivors, their siblings, and population controls.The project will provide new knowledge on childhood cancer etiology and childhood cancer late effects.