The common adverse effects of traditional nonsteroidal antiinflammatory drugs on renal function include reductions in renal blood flow, glomerular filtration rate, and sodium and potassium excretion, mainly via inhibition of renal cyclooxygenase. We designed the present study to determine the effects of IV paracetamol or parecoxib on renal function in elderly patients undergoing orthopedic surgery. Seventy-five patients (76 +/- 8 yr, mean +/- sd) undergoing hip replacement or surgery of the femoral shaft completed this randomized and placebo-controlled study. After their arrival in the postanesthesia care unit, patients received an initial dose of the study medication, paracetamol 1000 mg IV (n = 25), parecoxib 40 mg IV (n = 25), or saline IV (n = 25); subsequent doses were administered for the next 3 days. Opioids were provided as rescue medication. Blood and urine samples were collected before and after surgery, and markers of renal function were determined. During the first 2 h after the initial dose of parecoxib, creatinine clearance was slightly diminished (125 +/- 83 to 86 +/- 45 mL/min, P < 0.05), whereas no significant decrease of creatinine clearance was observed in the placebo and paracetamol groups. After all treatments, sodium and potassium excretion as well as urine albumin and alpha-1-microglobulin were transiently increased (group differences: not signicifant). In conclusion, glomerular and tubular functions were transiently affected in all patients after orthopedic surgery; however, the differences between the treatment groups were small and not clinically relevant. Further studies are warranted to determine adverse renal effects of longer-lasting therapy with these drugs, especially in patients with renal impairment or concomitant diseases.