This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This open-label Phase II cross-over study will test hypotheses that 1) Pulsed AB will induce higher levels of Hb F than HU alone, and 2) assessment of factors influencing erythropoietic rates may identify patient subsets who are more likely to respond to HU + AB. The study is open only to patients who have not had resolution of their symptoms with HU alone, and have a baseline Hb F of at least 2%. Patients will serve as their own controls. Clinical events and hematologic factors will be assessed in patients on HU alone for 6 months. AB therapy will then be added, (4 days, once/ month), and Hb F parameters and clinical events will be assessed on combined therapy. If additive effects are found with the two agents, the trial may provide a basis for selecting patients most likely to benefit from AB (in addition to HU therapy) and for design of Phase III trials. (This small trial will not serve as a pivotal trial for FDA approval of AB for treatment of sickle cell disease.)