Percutaneous renal biopsies of native kidneys were performed in 394 adult patients (aged > or = 15 years) at Rush-Presbyterian-St Lukes Medical Center between February 1983 and April 1995. All biopsies were performed with the use of real-time ultrasound; the last 169 were done with the use of an automated biopsy needle. All patients had a normal bleeding time and were observed for 23 to 24 hours postbiopsy. A biopsy-related complication occurred in 52 patients (13%). Minor complications, defined as gross hematuria, or perinephric hematoma that resolved without the need for transfusion or intervention, occurred in 26 patients (6.6%). Major complications, defined as those requiring a transfusion, invasive procedure, or resulting in septicemia, also occurred in 26 patients (6.6%). Patients with complications did not differ from those without complications at the time of biopsy with respect to age, blood pressure, serum creatinine, or bleeding time, but did have a lower prebiopsy hemoglobin level (11 +/- 2 g/dL v 12 +/- 2 g/dL; P < 0.05). No significant difference in age, blood pressure, serum creatinine, or hemoglobin level at the time of biopsy was observed in patients with major complications compared with those with minor complications. The time interval between the procedure and the identification of a complication was available for 44 patients (85%): 24 (92%) had a major complication and 20 (77%) had a minor complication. The complication was apparent within 24 hours in all but one patient (98%). Overall the complication was identified in only 52% of patients at < or = 4 hours, 77% at < or = 8 hours, and 95% at < or = 12 hours. Major complications were identified in 46% of patients at < or = 4 hours, 79% at < or = 8 hours, and 100% at < or = 12 hours. In patients with major complications the postbiopsy hemoglobin decreased by 3.2 +/- 1.8 g/d; this was significantly greater (P < 0.0001) than the decrease seen in patients with minor complications (1.2 +/- 1.0 g/dL) or in patients with no complications (0.9 +/- 0.8 g/dL). When considering a percutaneous renal biopsy as an outpatient procedure, observation of patients for 23 to 24 hours is optimal. Observation for 8 hours or less risks missing > or = 20% of complications.