Introduction - Following the simultaneous occurrence of tuberculosis cases at two high schools in Seine-Saint-Denis, the department’s tuberculosis control center conducted a study aimed at identifying the transmission dynamics and demonstrating the value of active case-based screening. Method - This retrospective study was conducted during the second half of 2010 at the department’s screening centers. Data were collected regarding screening efforts around the three initial index cases, diagnosed following the onset of symptoms, as well as around secondary cases, diagnosed following organized screening. Genotyping of a subset of the bacillus strains was performed. Results - Around the three index cases, 1,347 people underwent comprehensive screening (63% of identified contacts); 15 cases of active tuberculosis (1.1% of the 1,347 people screened) and 229 cases of latent tuberculosis infection (17.0% of the 1,347 people screened) were diagnosed. A link was established between the screening efforts at the two high schools. Three different strains of the bacillus were identified. Discussion and Conclusion - The study confirmed the benefits of implementing rigorous case-based screening, which yield both individual and collective benefits. It also identified factors that may contribute to the continuation of transmission chains: the sometimes inadequate training of medical and paramedical staff regarding the disease, the resulting errors in care, and the organizational challenges and issues of acceptability associated with contact tracing. (R.A.)