Objective To analyze the urine of renal recipients for the presence of donor DNA in an attempt to establish an altative diagnostic means of acute rejection.Methods Sixty-four renal transplant recipients were examined. Thirty-seven were norma lafter transplantation, while 22 others developed acute rejection, based on ser um creatinine levels and/or needle biopsy findings of the graft. Five developed drug-induced renal dysfunction. In female recipients with a male graft, we ex amined urine for the presence of Y chromosome (SRY and DYZ-1) and in recipients receiving an HLA mismatched graft, we looked for HLA-DR gene (DRB1) using PCR .Results Among the 14 female recipients with male grafts demonstrating stable renal function, only one was positive for SRY and DYZ-1 on the Y chromosome. However, SRY and DYZ-1 were found in the urine of four female patients with acute rejection , but these DNA fragments were not detected in 3 of the 4 after anti-rejection therap y. The last patient was referred to hemodialysis. Of 23 recipients of a graft from HLA mismatch donors with stable renal function, DRB1 was negative in 21 (91 %). Of 18 patients with acute rejection, DRB1 was positive in 16 (89%) and nega tive in 2. These DNA fragments were no longer found in 13 patients after anti -rejection therapy. In all patients with drug induced renal dysfunction, donor -derived DNA was negative.Conclusions Presence of door specific DNA in the urine of the recipient is strongly associat ed with acute rejection. Analysis of DNA derived from donor cells in urine was an effective and accurate method for the diagnosis of acute rejection of a renal transplant.