The role of de novo donor-specific alloantibodies (DSAs) in renal allograft injury is still unclear. The aims of this study were as follows: to assess the development of DSAs during the first year after transplantation, to determine the cause of DSA production, and to evaluate the association of DSA with allograft function. The study included 78 consecutive transplant recipients with negative cross-matches before transplantation. Recipient serum samples were assayed for DSA at 2 weeks as well as at 1, 3, 6, 9, and 12 months using a complement-dependent lymphocytotoxic (CDC) cross-match technique with donor lymphocytes. Among 545 cross-match tests performed after transplantation, there were 79 positive results. DSA appeared de novo in 44.8% of recipients: in 20 patients at 2 weeks; in 23 patients at 1 month; in 14 patients at 3 months; in 9 patients at 6 months; in 5 patients at 9 months; and in 8 patients at 12 months. Between month 3 and 9 after transplantation, DSA disappeared in 22 patients and appeared in 11 others. In 20 patients (57.1%) the appearance of DSA was associated with an acute rejection episode. In 11 of these, C4d deposition was found. In comparison with 43 patients without DSA, the serum creatinine levels during the first year after transplantation were significantly higher among patients with DSA. Transplant recipients produce antidonor alloantibodies. The highest rate occurs during the first month with the incidence diminishing at 3 months after transplantation. The development of DSAs in more than half of the patients was associated with rejection episodes. Patients with antidonor alloreactivity showed worse renal function.