Although polyvalent pneumococcal vaccine and prophylactic penicillin are used to prevent overwhelming Streptococcus pneumoniae septicemia in infants and young children with sickle cell anemia, infection rates remain high. We have reviewed our seven-year experience with a regimen of twice daily oral penicillin V potassium prophylaxis in 88 affected children. The median age at the start of prophylaxis was 10 months, and the median duration of prophylaxis was 29 months (range, three months to seven years). The total period of observation of patients who were prescribed penicillin was 248 person-years. Most patients also received one or two doses of polyvalent pneumococcal vaccine. Despite penicillin prophylaxis and pneumococcal vaccine, eight episodes of S pneumoniae septicemia have occurred and three have been fatal. Four episodes were in children older than 3 years. Suboptimal compliance with the prescribed oral penicillin regimen was usually apparent. With one possible exception, the infections occurred when penicillin had not been taken during the previous 24 hours. The S pneumoniae septicemia rate in this patient population, 3.2 per 100 person-years, is somewhat less than that described in previous reports of children not receiving penicillin but is still unacceptably high. Vigorous advocacy of a penicillin prophylaxis regimen does not eliminate the risk of pneumococcal septicema in this patient population.