Microchimerism may play a part in transfusion complications. The aim of this study was to see if the establishment of post transfusion microchimerism was dose dependent.Methods: Twenty non-pregnant female patients, without known malignant or immunological diseases, mean age 68 years, receiving 2-4 (1-2 from a male donor) red blood cell concentrates (RBCC) during elective surgery, were included. Ten patients received buffy-coat depleted (BCD) RBCC, leukocyte count 10E8 – 10E9 per unit; and ten received RBCC leukoreduced (LR) by prestorage leukocyte filtration, leukocyte count <10E6 per unit. EDTA samples collected in vacuum tubes before, one week and 6 months after transfusion, were frozen and stored at –40 C. Genomic DNA was isolated and PCR performed using four primer sets amplifying markers on the Y chromosome.Results: Microchimerism was detected in a total of eight out of thetwenty patients. In three patients microchimerism was detected beforetransfusion. These patients had given birth to 1-2 boys each, and had no history of previous transfusion. Two patients receiving BCD RBCC and two patients receiving LR RBCC had detectable microchimerism one week after transfusion. Age of the transfused RBCC was 6, 24, 8 and 7 days respectively. One patient receiving LR RBCC had detectable microchimerism after six months. The age of this unit was 22 days. No patient had detectable microchimerism in more than one blood sample.Discussion: In this study we demonstrate that microchimerism aftertransfusion does not seem to be dose dependent, and can be induced even by >3 weeks old LR RBCC.