The results of the meningococcal vaccination campaign carried out in Cantabria in February-March 1997 are presented. In the short term, the campaign reduced the rate of incidence in the risk group from 21.33/100,000 in the previous 12 months, to zero in the 12 months following the campaign. In the medium term (2 and a half years later), the rate of incidence was maintained 80% lower in the risk group and 77.3% lower in the population overall. The vaccinal efficacy in the third year following the campaign remained above 91% for all age groups, although some biases commented upon in the study have meant that this efficacy has been underestimated. Also, the vaccinal efficacy is much higher than the percentages of seroprotection measured by the rate of bactericidal antibodies, particularly in the younger age groups. The clinical cases in vaccinated individuals experienced a benign clinical course.