Alloimmunization to red cell antigens contributes to morbidity in transfused patients. It has been recommended that blood for sickle cell patients need not be matched for antigens other than ABO and Rh(D), as there is no greater incidence of antibody production than in other multitransfused patient populations. Post transfusion alloimmunization was studied in a group of 34 sickle cell disease patients attending a U.K. haemoglobinopathy clinic. Red cell antibodies were formed in 17.6% of the transfused patients and Rhesus and Kell antibodies accounted for 66% of this total. In order to reduce alloimmunization, a policy of performing extended red cell phenotyping on the patients, and providing blood matched for Kell, and in certain circumstances the Rhesus antigens other than Rh(D), is recommended.