Objective: Cancer-registry data are crucial for definingcancer incidence rates for use in setting service priorities and monitoringservice effects. This applies in Thailand where cancer is the leading cause of death and service needs are high. The Bangkok Cancer Registry (population-based) was established in 1990to determine cancer incidence rates for Bangkok. This proved difficult, however, because the Bangkokpopulation (>8million) fluctuates with numbers of temporary visitors, many of whom visit Bangkok temporarily for services. If these visitors are mis-categorized as usual residents, cancer incidence rateswould be inflated. During 2013-2015, residential addresses on the Registry were cross-checked against official addresses on the National Civil Registration records of the Ministry of Interior. The effectsof this cross-checking on incidence rates are discussed. Methods: Residential addresses recorded on the Registry for cancer diagnoses in 2013-2015 were corrected using official Ministry data. Effects on numbers of recorded cancers and crude and directly age-standardized rates (World Population) were determined. Results: Of 44,813 cancer casesdiagnosed and recorded on the Registryduring 2013-2015, 36,327 (81.1%) had an official Bangkok address. When limiting analyses to these cases, the crude incidencefor all cancer sites combined reduced by 18.9% (19.7% for males and 18.3% for females). Corresponding reductions in age-standardized incidence rates were 20.0% for males and 18.8% for females. These reductions varied for common cancer sites:in males,from 14.8% for lung to 25.9% for colorectal cancer; and in females, from 12.9% for lung to 24.0% for cervical cancer. Conclusions: These differences are considered sufficient in magnitude to justifyroutine use of official residential data when calculating cancer incidence rates for Bangkok. If these rates are to be compared with comparable rates for other Asian citiesthat serve broader populations, equivalent methodologies for determining residential status would be needed for all cities.