Streptococcus pneumoniae is the most frequent bacterial cause of pneumonia, otitis media, and bacteremia and is the third most common cause of meningitis in infants and children. Infants have the highest rates of pneumococcal disease. Recent studies showed that the peak incidence of meningitis occurs among infants three to five months of age, that of otitis media occurs among infants six to 12 months of age, and that of hospitalization for pneumonia occurs among infants 13 to 18 months of age. A predominance of males was noted in most studies of pneumococcal disease. Blacks have higher rates of bacteremia and meningitis than do whites; this difference is explained only in part by the greater susceptibility to infection of children with sickle cell disease. However, the incidence of otitis media is lower in blacks than in whites. The predominant pneumococcal serotypes change with age, with time, and with geographic location. Certain serotypes are responsible for most cases of bacteremia or meningitis. Types present in the pneumococcal vaccine currently available represent 85% to 97% of the strains that have caused disease in the United States in recent years.