Background: The retrospective study of cancer incidence around the Gilly-sur-Isère incinerator (Savoie, France)—initiated amid a crisis following its closure in late 2001—aimed to determine whether there was an excess of cancer cases in the vicinity of the facility. In the absence of a cancer registry in Savoie, the study involved compiling as comprehensive a list as possible of cancers occurring between 1994 and 2002 in the study area (exposed to atmospheric emissions from the incinerator) in order to compare the observed incidence with that established by French cancer registries. The objective of this article was to describe the difficulties encountered and the solutions implemented during the cancer survey; the results of the incidence study are not presented. Methods: Medical data were collected from multiple sources: cytopathology and hematology laboratories, medical records departments of hospitals and clinics, primary health insurance funds, private practitioners, and specialized cancer registries. The collected electronic files were processed: identification of missing addresses, selection of patients from the study area, standardization of cancer codes, merging of files into a single database, analysis of available information on each cancer, and deduplication. The majority of cancers were then validated by reviewing medical records to eliminate false positives (metastases from a known primary cancer, recurrences, etc.). Results: Of the 2,845 cancers initially identified from the electronic data, 28% were excluded because they did not meet the case definition (lack of evidence of cancer, diagnosis date prior to the study period, etc.); ultimately, the database included 2,055 cancer cases. The calculated quality indicators showed that the database had completeness and validity comparable to those of registries. Three main types of sources enabled the identification of 94% of cases: laboratories, hospital medical information departments, and primary health insurance funds. Conclusion: The use of administrative data and the review of medical records proved necessary given the uncertainties regarding patients’ addresses at the time of diagnosis, errors in cancer coding in some of the collected files, and difficulties in identifying false positives. Consequently, significant human resources had to be committed to this census.