Background and Objectives: The National Mesothelioma Surveillance Program (PNSM) was launched in 1998. The program’s primary objective is to estimate the incidence of pleural mesothelioma and its trends in the French population, as well as to analyze occupational and non-occupational risk factors for this disease. The PNSM is based on a "comprehensive" registry of mesothelioma cases in a number of French departments (17 in 1998, 21 in 2003). Based on this registry, a national incidence estimate is calculated by cross-referencing with national pleural cancer mortality data from the CépiDc at INSERM. The medical-administrative database known as the Program for the Medicalization of Information Systems (PMSI) provides insights into the activity of public and private healthcare facilities through a registry of hospital stays. At the national level, an aggregated database is compiled anonymously: the database of anonymous discharge summaries (RSA), which does not allow for the identification of patients. This nationwide database contains information on diagnoses and reasons for hospitalization for each recorded stay, which could potentially supplement the data typically collected to estimate national incidence and prevalence rates. The study presented here focused on examining the concordance between data recorded in 1998 and 1999 by the first 17 departmental mesothelioma registries of the PNSM and those recorded in the PMSI RSA for the same period and geographic coverage. Method: The first step involved identifying in the PMSI database any multiple hospital stays for patients with a diagnosis suggestive of pleural mesothelioma residing in the departments covered by the PNSM. An estimate of the prevalence rate of cases recorded in 1999 in this database was also performed. The PMSI data were cross-referenced with those of the PNSM. Given that the current structure of the national PMSI database does not allow for direct identification of patients, indirect matching was performed. This matching was initially carried out using the strictest available criteria (age and municipality of residence), and then, for the remaining subjects, using broader criteria (age and department of residence). To validate the entire procedure, individual validation by cross-referencing with the patient’s medical records was conducted in one department (Loire-Atlantique) with the assistance of the Medical Informatics Departments (DIM) of the relevant institutions. Results: Despite the difficulties, the methods for grouping potential multiple hospital stays for the same patient proved effective, particularly due to the rarity of this condition: 506 eligible cases were identified in 1998, including 230 with a diagnosis of pleural mesothelioma among the 1,263 selected hospital stays, and 474 eligible cases, including 232 with a diagnosis of mesothelioma, among the 1,413 stays in 1999. Among these, cross-referencing with PNSM data—strictly based on municipality of residence—allowed for the identification of 70% of subjects in the PMSI in 1998 and 68% in 1999. Extending the search to cases not pathologically confirmed in the PNSM allowed for the identification of 63% and 69% of subjects in the PMSI, respectively. Case-by-case validation conducted in one department by reviewing patient records covered 203 hospital stays. This confirmed that, on the one hand, the process of deduplicating hospital stays had been generally satisfactory, with the exception of a single patient who was not identified due to an incorrect gender in one of the PMSI records, and two patients who were mistakenly grouped into a single entry due to the proximity of their birth dates. The matching with cases recorded in the PNSM was excellent when the city of residence was entered in both databases; when this data was missing, eight out of ten times the correct patient was found among the possible matching combinations. Furthermore, this comparison identified 3 cases that were not known to the PNSM. The diagnoses recorded in the PMSI were accurate (pleural mesothelioma) for 15 out of 18 confirmed cases in the PNSM; the concordance of diagnoses recorded in the PMSI compared to medical records shows fairly good agreement, with only a small proportion of hospital stays coded as mesothelioma corresponding to pleural metastases. However, a significant proportion (nearly 40%) of hospital stays recorded in the PMSI corresponded to patients whose diagnoses were previously known, i.e., prevalent cases. As things stand, it does not seem reasonable to rely on the PMSI as the sole source of data for estimating mesothelioma incidence, particularly given the high number of prevalent cases recorded and errors in diagnostic coding. The issue of duplicate hospitalizations for the same patient should be resolved in the coming years through the introduction of a system to link patients’ hospital stays. Furthermore, the consolidation of national data makes it available after two years. Nevertheless, it constitutes a source of supplementary information and should be systematically consulted and regularly cross-referenced with data from mesothelioma registries at the departmental level. (R.A.)