Renal transplant biopsies to diagnose transplant pathology are routinely performed using ultrasound guidance. Few large studies have assessed the rate and risk factors of major biopsy complications. This study is a single-center 5-year retrospective cohort analysis of 2514 biopsies. Major complications occurred in 47 of 2514 patients (1.9%) and included hospitalization, transfusion of blood products, operative exploration and interventional radiology procedures. The complication rate among "cause" biopsies was significantly higher than in "protocol" biopsies (2.7% vs. 0.33%, p < 0.001). Complications presented on postbiopsy days 0-14, with the majority diagnosed on the same day as the biopsy and manifested by hematocrit drop, although the presence of such delayed presentation of complications occurring >24 h after the biopsy on days 2-14 is previously unreported. Specific patient characteristics associated with increased risk of a complication were increased age and blood urea nitrogen, decreased platelet count, history of prior renal transplant, deceased donor transplant type and use of anticoagulant medications but not aspirin.