Occupational asthma is one of the most common work-related respiratory diseases in industrialized countries. The National Observatory on Occupational Asthma (ONAP) was established in 1996 through a joint initiative of the French Society of Occupational Medicine and the French-Language Society of Pulmonology. Data collection relied on the voluntary participation of pulmonologists, occupational pathology consultants (CPP), and occupational physicians. To explore the possibility of using ONAP data for epidemiological surveillance, a pilot study was launched in 2003 in collaboration between the French Institute for Public Health Surveillance (InVS) and the Interuniversity Institute of Occupational Medicine of Paris Ile-de-France (IIMTPIF). The objective was to assess the feasibility of reconstructing the source population from which cases of occupational asthma referred to the CPPs by occupational physicians originated, in order to calculate incidence rates. This source population consists of all employees monitored by the occupational physicians who referred the cases to the CPPs. To reconstruct this population, it was necessary to identify the SIRET codes of the establishments monitored by occupational physicians. The breakdown of employees by sex, age, industry sector, and occupation was provided by INSEE based on data from the "Annual Declaration of Social Data" (DADS) database. The 57 participating occupational physicians (68% participation rate) reported 58 new cases of occupational asthma between October 2003 and April 2004. The number of establishments they reported monitoring was 12,719. After verification, the SIRET code of the establishment could be identified for 8,542 (67%), totaling 223,095 monitored employees. The estimated incidence rate over the 7-month study period was 260 cases per million employees. Overall, this study demonstrated that it is difficult to reconstruct a source population retrospectively. For this reason, a project to comprehensively collect data on new cases of occupational asthma in defined geographic areas is being developed (Onap II project). (R.A.)