The major cause of death in sickle cell anaemia is from infection, especially infection caused by Streptococcus pneumoniae. Meningitis, pneumonia and septicaemia caused by this organism are the primary types of infection leading to death. Children under three years of age are at highest risk. We have known for over twenty years that approximately 30 per cent of the infants born with sickle cell anaemia will become infected in the first three years of life and one-third can be expected to die from the infection. These data were the reason that we conducted the Prophylactic Penicillin Study (PROPS), a trial to investigate the effectiveness of oral prophylactic penicillin in preventing severe infection due to S. pneumoniae. This investigation was a very efficient, cost effective study because of its timeliness and its conduct within the framework of an ongoing study. Moreover, the question being answered was simple and focused with up-to-date data that permitted accurate estimates of sample size and incidence.