The report describes the feasibility of the addition of multiple viable HLA-mismatched unrelated cord blood units, to a low cell number matched unrelated cord, to assist clinical engraftment. An ablative stem cell transplant was performed in an adult with relapsed acute lymphoblastic leukaemia (ALL), using a single HLA-matched cord blood unit (mononuclear cell dose 0.8 × 10(7)), supported by six mismatched cord blood units (one unit per 10 kg recipient weight). No adverse reaction occurred following the infusion of mismatched units and engraftment of the suboptimal-dose matched unit occurred rapidly, with no molecular evidence of engraftment of mismatched cords. Early molecular remission of ALL was demonstrated using a novel PCR for a mitochondrial DNA mutation in the leukaemic clone. The cell dose of the matched cord was well below that recommended to engraft a 70 kg recipient. We suggest that a factor or factors in the mismatched cords enhanced/supported engraftment of the matched cord.