Background.Early detection of rejection after lung transplantation may prevent allograft failure. This study determines if mRNA from the cell adhesion molecules intercellular adhesion molecule-1, vascular cell adhesion molecule (VCAM)-1, and E-selectin in pulmonary endovascular tissue samples could be markers of early rejection.Methods.Single left lung transplants were performed in five dogs. Each dog was treated for 2 weeks with immunosuppression, after which rejection was allowed to occur. Percutaneous biopsies from 2- to 3-mm distal branch pulmonary arteries were obtained in each dog from the normal and the transplanted lungs at the end of immunosuppression therapy and periodically (2–4 times) for 1 to 3 weeks until euthanasia. Levels of cell adhesion molecule mRNA in the biopsy samples were quantitated by reverse-transcriptase polymerase chain reaction and normalized to &bgr;-actin mRNA levels.Results.Between three and five pulmonary endoarterial biopsy samples were obtained from each lung at each catheterization procedure. There was a significant increase in VCAM-1 mRNA levels in the biopsies of the transplanted lungs (which were undergoing rejection) compared with the native right lungs in all dogs. Progressive increases in VCAM-1 mRNA were observed with longer rejection times. VCAM-1 mRNA changes were detected earlier than histologic changes of rejection.Conclusions.In pulmonary endoarterial biopsy samples obtained in a canine lung transplant model, there was a progressive increase in VCAM-1 mRNA levels with increasing rejection. Changes in VCAM-1 mRNA were observed earlier than histologic changes of rejection. VCAM-1 quantitation by endoarterial biopsy may be useful in surveillance and early diagnosis of rejection in patients who undergo lung transplantation.