Field trials in Papua New Guinea have shown that pneumococcal capsular polysaccharide vaccine protects children less than 2 years of age from death due to acute lower respiratory infections (ALRI). The vaccine appears to reduce mortality by preventing bacteremia. Evidence suggests that highly invasive "adult" pneumococcal serotypes are comparatively more important as a cause of ALRI in developing countries where ALRI-related mortality is higher than in North America. Capsular polysaccharide vaccines are more likely to be immunogenic for these serotypes than for the "pediatric" serotypes. The evaluation of pneumococcal vaccines in areas where mortality is high should have the highest priority. For such trials investigators should, at minimum, define epidemiologic circumstances in terms of (1) the invasive serotypes of pneumococci, (2) the protective levels of antibody, (3) the antibody response to vaccination, and (4) the general immune status of the population.