In the winter of 1998-1999 an outbreak of pneumococcal pneumonia occurred among Ranger students undergoing high-intensity training. Thirty pneumonia cases (attack rate = 12.6%) were identified among a group of 239 students. Eighteen students were hospitalized; Streptococcus pneumoniae-positive cultures were detected in 11 (61.1%) of these 18 hospitalized cases. Pneumococci were also identified in throat swabs of 30 (13.6%) of 221 nonhospitalized students surveyed. Serum antipneumolysin seroconversions were detected in 30 (18.3%) of 164 students tested. An association between development of serum antipneumolysin antibody and pneumococcal pharyngeal carriage/colonization was found. Of 30 seroconverters, eight (26.7%) had S. pneumoniae-positive cultures compared with only 17 (12.7%) of 134 nonseroconverters (relative risks = 2.02, 95% confidence interval = 1.02-4.02, p = 0.05). The outbreak was controlled by administrating lowdose, oral azithromycin prophylaxis (250 mg weekly for 2 weeks) and was associated with a 69% reduction in pneumococcal carriage and a 94% reduction in pneumonia rates.