Core biopsies have been done by ultrasound assisted 18-G disposable needles with a spring loaded gun (Biopty) system in 140 renal transplant cases either for investigation of an early non-functioning graft or evaluation of deteriorating graft functions. The biopsy procedure was successfully completed in 99.5% and sufficient amount of renal tissue was obtained in 88% of cases. The pathological diagnoses were confirmed 100% by the other clinical parameters of cases with acute cellular rejection, pyelonephritis, acute tubular necrosis and there was disease recurrence. In another 8 patients (6%) where the pathological picture was showing either no or nonspecific changes there was no major change in clinical outcome. In addition, clinical diagnoses of chronic vascular rejection and Cyclosporin A toxicity were confirmed in 93.7% and 91.7%, respectively, in biopsies of these cases. Complications were seen in 3 patients as a bowel perforation, intra-abdominal bleeding and formation of an intrarenal arterio-venous fistula. In former two complicated cases there was no need for any extra treatment but the arterio-venous fistula was successfully embolized through an angiography catheter without losing the graft. We conclude that the Biopty system is more efficient than the fine needle aspiration biopsy especially when the pathological diagnosis can be made upon tissue components rather than cells alone.