- Streptococcus pneumoniae is an important cause of pediatric morbidity and its main reservoir is the nasopharynx, from which it can disseminate and cause invasive disease. From November 1997 through March 1998, nasopharyngeal carriage of S. pneumoniae was evaluated in 250 children under the age of 36 months: 123 Jews and 127 Bedouins with acute respiratory disease and in 980 healthy control children (852 Jews and 128 Bedouins). Carriage rate was higher among sick children. Among Jewish children it was 57% and 35% of sick and healthy children respectively (p < 0.01), and among Bedouin children it figured as 80% and 67% respectively (p = 0.01). The difference in carriage rate was most prominent in infants under the age of 5 months: among Jewish children it was 60% and 27% of sick and healthy children respectively (p < 0.001) and among Bedouins it was 82% and 65% respectively (p = 0.05). Higher carriage rate of penicillin resistant pneumococci (PRP) was also detected in sick children, with no relation to antibiotic treatment in the month prior to sampling. In Jewish children PRP was detected in 12%, 28% (p < 0.001) and 36% (p < 0.001) of healthy children, sick children with previous antibiotic treatment and sick children with no treatment, respectively. The seroypes included in the newly developed 7-valent conjugate vaccine: 4, 6B, 9V, 14, 18C, 19F, 23F, that are highly pathogenic and often antibiotic resistant contributed 74% of isolates in sick Jewish children who had previous antibiotic treatment and 39% of isolates in healthy children (p < 0.001). In Bedouin children vaccine types carriers rate among the sick children was not higher than in healthy children. Acute respiratory disease increases the risk of pneumococcal carriage in general and carriage of resistant pneumococci in particular. Previous antibiotic treatment increases the risk of carring one of the pathogenic serotypes included in the 7-valent vaccine. The impact of disease is most prominent in infants under 5 months, since they are usually less exposed to S. pneumoniae carriers than older children. Since the increase in carriage rate during illness is mostly due to the serotypes included in the newly developed conjugate vaccine, future immunization programme may decrease not only morbidity rate but also nasopharyngeal carriage rate of pneumococci in general and of antibiotic-resistant pneumococci in particular.