We evaluated the change in the percentage of cells of donor origin in pleural fluid of 13 consecutive patients who underwent lung transplantation. Pleural fluid was sampled 2, 4, and 8 days after lung transplantation. DNA, which was extracted from the blood of donors and recipients and from the pleural fluid, was amplified using a polymerase chain reaction technique. The reaction products were electrophoresed, and bands indicating amplified human leukocyte antigen (HLA)-DR alleles were quantified by determining the area under the curve (AUC) by a densitometric analysis. HLA-DR alleles, which were present only in recipient cells (recipient allele), were analyzed and compared to HLA-DR alleles that were present only in donor cells (donor allele). A dilution study was first performed to provide a standard curve relating the percentage of donor and recipient cells in a mixture to their AUC. The AUC of the recipient alleles did not change significantly over the first 8 postoperative days. The AUC of the donor alleles was less on postoperative days 4 and 8 than on day 2 (p<0.05). The donor allele AUC on day 8 was <20% of the shared allele AUC, corresponding to <1% of all cells by the dilution study. We conclude that donor cells are rapidly cleared from the pleural space after lung transplantation, with <1% of cells of donor origin by postoperative day 8.