Population-based cancer registration plays a key role in malignant disease surveillance by providing data needed to calculate incidence, survival, cancer cluster investigations and prevalence trends. The Finnish Cancer Registry (FCR) maintains a national register of all cancer cases diagnosed in Finland and conducts epidemiological and statistical research on cancer [1]. The four main dimensions of cancer registry quality evaluation are validity, timeliness, comparability and completeness. Completeness is used to express the extent to which all incident cancers occurring in the population are included in the registry database [2]. Cancer registry data are utilized widely in cancer research and poor completeness of data introduces bias to incidence and survival estimates [3]. Rare diseases, such as childhood cancers, are sensitive to the effects of incomplete and/or inaccurate registration. Childhood cancer incidence has been reported to be higher in the Nordic countries compared to the rest of Europe [4]. This is thought to be explained by high completeness of the registers, as notification of new cancer cases is mandatory in all Nordic countries [5], and the Nordic cancer registries are reported to have close to 100% completeness for solid malignancies [6-10]. The survival rate of certain childhood cancer types (brain, lymphoma and acute lymphoblastic leukemia) in Finland was recently reported to be among the highest in the world [11].