During March and April of 1992, the health surveillance system began detecting increasing numbers of cases of meningococcal disease (MCD) in the Islamic holy city of Makkah (Mecca). We identified 102 bacteriologically confirmed cases (CC) and 80 suspected cases (SC) of MCD. Neisseria meningitidis was identified as Group A, III-1 clone. The ratio of male:female cases was 2.9:1. All age groups of males were affected. There was only one case among women aged 10-30; 50% of the adult female cases were 55 or older. The case-fatality ratio (CFR) was 14.7% among CC. Pakistanis, who comprised about one-third of the CC, had a CFR of 26.7%. Fifty-nine percent of CC were religious visitors. CC in residents were most common in persons living near the Holy Mosque (Haram), where the carriage rate reached 86%. A mass vaccination program against MCD was instituted, using AC bivalent meningococcal vaccine (MCV). An abrupt drop, from a mean of 15 CC per week to 2 CC per week (only in visitors), coincided with vaccinating 600,000 persons over 2 weeks. Makkah residents who had been vaccinated against MCD were less likely to have contracted MCD (OR = 0.17, 95% CI: 0.06-0.50). MCV was of no significant protective value if it had been administered 5 years before the outbreak. The main reason for not being vaccinated as stated by both cases (71%) and controls (45%) was not knowing about the disease. The age and sex differences probably relate to differences in exposures to crowded conditions. Health education should illuminate the seriousness of the disease and the importance of vaccination.