Mandatory reporting (MR) of invasive meningococcal disease (IMD) serves two purposes: to initiate prophylaxis among contacts of a case and to study the characteristics of the disease. The two main limitations of this surveillance method are incomplete reporting and delays in reporting. The authors’ objective is to determine the completeness of mandatory reporting in the Nord-Pas-de-Calais region in 1997 and 1998 by estimating the total number of IIMs using the capture-recapture method based on mandatory reports and hospital data. The capture-recapture analysis estimates the completeness of the DO in the region for the two years at 65%, which is comparable to the national average. A difference in completeness between the region and France as a whole does not explain the large difference in crude incidence between Nord-Pas-de-Calais (1.02 per 100,000 inhabitants per year in 1997–98, calculated based on cases meeting the criteria of the 1990 circular) and France (0.64 per 100,000 in 1998). In practical terms, this means that in 35% of cases (more than 25 cases per year in the region), we are not certain that community-wide prevention measures have been implemented, even though a number of these cases are of an age to be in group settings (50% were between three and eighteen years old; the median age was seven, compared to four for cases reported under the 1990 circular). This situation could improve with the new reporting criteria, which are broader than those in the 1990 circular. (adapted from the text)