The development of a national surveillance system, the Multi-Source Cancer System (SMSC), to complement the registry-based system was recommended by the Commission for the Assessment of the Health Consequences of the Chernobyl Accident and endorsed by successive government cancer plans. The principle is based on cross-referencing anonymized data from three sources: the Medical Information Systems Program (PMSI), long-term conditions (ALD), and pathology reports (ACP). The objective of the pilot study was to assess the feasibility of implementing the SMSC. It focused on thyroid cancer in two regions: Île-de-France and Nord-Pas-de-Calais. Data collection from the ACP reports was conducted retrospectively (2004) using a computerized form. In total, 423 PBC data sheets for thyroid cancer were submitted by volunteer pathologists from both regions. These were cross-referenced with data from the ALD (1,159) and the PMSI (2,947) for the year 2004. The matching rate (number of ACP cases found in the other sources) observed was 84% (all 3 sources), 81% (ACP, PMSI), and 48% (ACP, ALD), respectively. The implementation of the system highlighted the fact that the chosen IT solution was too complex for nationwide deployment. This complexity stemmed from the technical difficulties encountered during the project. At the conclusion of the pilot, two areas for developing the SMSC at the national level were identified: - in the immediate term, cross-referencing the PMSI and ALD sources for descriptive purposes and to produce surveillance indicators; - in the medium term and in parallel with the first area, studying the conditions for establishing the national ACP database. (R.A.)