The Danish Cancer Registry (DCR) and the Danish Lung Cancer Registry (DLCR) are nation-wide registries recording Danish patients with lung cancer (LC). The aim of this study was to assess data agreement and possible consequences hereof on estimation of survival between patients in the two registries.Descriptive statistics were used for comparison of registered patients in 2013-2014 in the DCR and the DLCR. Furthermore, the one-year relative survival (1y-RS) and Cox proportional mortality hazard rates (MRR) were calculated.In 2013-2014, a total of 9,111 Danish residents were identified with LC in the DCR and 9,316 were found in the DLCR. Merging the two registries showed an agreement of 87%, whereas 6% were included only in the DCR and 8% only in the DLCR. Including patients only registered in one registry, but who seemed to meet the inclusion criteria of both registries, would increase the agreement to 95%. No differences were seen for 1y-RS. However, MRR for patients in the DLCR was significantly lower than for patients in the DCR: 0.94 (95% confidence interval: 0.91-0.98).Surprisingly, the DCR registered fewer patients in 2013-2014 than the DLCR, even though they employ the same primary data source. The agreement between the DCR and the DLCR was 87%; this may be increased to 95% if patients who seemed to meet the inclusion criteria of the other register were also included. The discrepancies found were mainly due to different definitions of dates of diagnosis, registrations probably missed by the algorithms and possible registration errors. Discrepancies resulted in a significant difference in MRR, but not in 1y-RS.none.not relevant.