Several complications can occur during both the early and late postoperative periods after kidney transplantation. The methods used to follow up 575 kidney transplanted patients, (transplantations performed between October 1979 and November 1997) in the early (within 6 weeks) and late postoperative periods have been assessed. The diagnostic value of core biopsies and ultrasound examinations, the prevalence of complications, and the applicability of the diagnostic tools in the evaluation of the graft status and viability were analyzed. In the early postoperative period, graft rupture occurred more frequently after biopsy than in the late period (7.4% vs 0.82%), this leading graft loss in 18 of 20 cases. The sonographically diagnosed morphologic and functional changes were also analyzed. Sonography proved a very accurate method for the detection of perirenal fluid collections and masses and severe vascular complications. The data demonstrated that biopsy is indicated in the early postoperative period when the result of sonography is doubtful. In the late postoperative period, biopsy should be performed in every case.