The development of antihuman leucocyte antigen antibodies (aHLAA) in response to multiple transfusions in preterm infants was studied prospectively. Fifty seven infants requiring a minimum of two blood transfusions were recruited after obtaining informed written parental consent. They were randomised to receive either whole blood or blood that had been passed through a leucocyte filter. Anti-HLAA were sought in maternal and cord blood so as to ensure that any aHLAA detected after transfusion had not been passively transferred antenatally, and in 1 ml samples drawn monthly from the baby, at least 10 days from a previous transfusion, until discharge from hospital. Anti-HLAA were detected by microlymphocytotoxicity assay. Results were obtained in 42 babies, 19 in the filter and 23 in the no filter group. Fifteen babies had to be excluded because of protocol violation or because they died. None of the babies receiving filtered blood developed aHLAA, but seven babies in the no filter group developed aHLAA. In conclusion, multiply transfused preterm infants have the ability to elaborate antibodies to HLA and leucocyte filters may prevent this.