Streptococcus pneumoniae is a leading cause of bacteremia, pneumonia, meningitis, and otitis media in the United States. Prior to 1987, this organism was uniformly susceptible to penicillin; since then, however, increasing numbers of isolates resistant to penicillin, as well as to other common antibiotics, have been identified. Frequent exposure to antibiotics has been documented as an important risk factor for the emergence of resistant organisms in HIV-infected patients, who are more likely than uninfected people to be colonized with antibiotic-resistant strains of S. pneumoniae. This substudy is the first to examine the effects of withdrawing or delaying the initiation of prophylaxis (in this case, MAC prophylaxis) on the prevalence of antibiotic-resistant pneumococci in a prospective manner.