ACH YEAR IN THE UNITED STATES, approximately 2000 children are born with sickle cell disease (SCD). 1 The incidence of invasive Streptococcus pneumoniae infection is 20-to 100-fold higher in children with SCD than in the general population. 2,3 Penicillin prophylaxis for children with hemoglobin SS reduces the incidence of invasive pneumococcal infection by 84%, independent of pneumococcal immunization status. 4 According to recent American Academy of Pediatrics (AAP) consensus statements, all children with hemoglobin SS and S-°thalassemia should take daily antibiotics to provide pneumococcal prophylaxis from age 2 months, or the time of their diagnosis, through their fifth birthday. 5,6 Achieving prophylaxis in children with SCD requires 2 steps: children must obtain antibiotics prescribed by a clinican, and they must take the medication as directed. The second step has been examined in several studies of patient adherence with penicillin prophylaxis using a variety of techniques. Parents have reported their children's compliance to be 61% to 67%, 7-9 and computerized pill bottles have measured 67% adherence with prophylaxis. 10 However, urine testing, which detects penicillin taken in the prior 15 to 24 hours, has revealed compliance varying from 47% to 64%. 8,9,11,12 These 6 studies were conducted at single institutions, included patients older than 5 years, and were subject to selfreporting bias or the artificiality of research conditions. Furthermore, to