- To determine recent trends in its epidemiology and the need to reconsider prophylactic interventions, meningococcal disease in the Israel Defense Force (IDF) from 1975 through 1993 was studied. All cases of meningitis or meningococcemia were included. A considerable increase in the number of cases has been observed since 1991, with serogroup C becoming predominant (76% of cases) since then. Serogroup Y was the second most frequent serogroup during this period, while serogroup B, predominant in the civilian population of Israel, was rare. Most cases occurred during the first 6 months of military service. Seasonality was important, with most of the cases occurring between December and March, although a small summer peak was also noted. Since 1992, three small clusters of meningococcal disease were encountered in the IDF, for the first time, with all cases caused by group C meningococci. In one cluster, the emergence of rifampicin resistance resulted in failure of chemoprophylaxis. The rise in group C and Y cases since 1991, and the occurrence of rifampicin resistance, necessitate considering meningococcal vaccines and new antimicrobial agents for prophylaxis.