This study included extensive laboratory tests in 783 patients with meningitis who were admitted to the Abbassia Fever Hospital, Cairo, between January 1971 and May 1974, and investigations of inapparent pharyngeal infections of patients' contacts, outpatients, and selected groups of school children. Almost half of the patients (380) were cerebrospinal fluid (CSF) culture-positive for Neisseria meningitidis serogroup A. Another 6% (46) were diagnosed as having meningococcal meningitis on the basis of microscopic evidence or demonstration of meningococcal polysaccharide antigen by counterimmunoelectrophoresis (CIE). Of the remaining patients, 7% (58) were infected with other pathogens, and in 38% (299) the etiology remained unknown. In a comparison of diagnostic methods, CSF culture and CIE identified equal percentages (about 83%) of meningococcal infections, while microscopic examination was less effective (about 69%). The CSF from meningococcal patients was generally free of viable meningococci and detectable antigen 48 hours after the initiation of ampicillin therapy. Although pharyngeal infection was highest among the CSF meningococcal culture-positive patients, 39% had negative pharyngeal cultures and 90% had negative blood culture. Inapparent pharyngeal infection ranged from 17–41% in CSF culture-negative patients with meningococcal meningitis, outpatients, and various categories of patients' contacts. Pharyngeal infection was somewhat lower in patients with meningitis of unknown etiology (15%), and quite low (2%) in meningitis patients infected with other pathogens. There was an inverse correlation between socioeconomic level and meningococcal carrier rates in school children. The great majority of the pharyngeal N. meningitidis isolates from contacts of nonmeningococcal meningitis patients, outpatients, and school children were not serotype A. Inapparent pharyngeal group A meningococcal infection occurred with significant frequency only in populations closely associated with group A disease.