A RECOGNIZED COMPLICATION: Ritonavir is an antiprotease used in the treatment of HIV-positive patients. Among the known side effects, nephrotoxicity can be severe. We have observed acute renal failure in 8 patients. CIRCUMSTANCES: Renal failure occurs early after introducing ritonavir (3-21 days). It is often severe with major creatinine elevation. One patient was dialyzed for 16 days. In these patients, saquinavir was usually associated with ritonavir. RITONAVIR ALONE: We retrospectively analyzed creatinine levels in 87 patients treated with ritonavir without saquinavir. Twelve of these 87 patients (13.7%) developed renal failure. Creatinine clearance (Cockcroft) was reduced 116 to 71 ml/min in 12 patients. Finally, it was demonstrated in 6 patients that ritonavir can reduce creatinine clearance by 25% after only 3 days of treatment. VIGILANCE: Ritonavir has a known nephrotoxic potential. Acute renal failure may be severe and can occur with ritonavir alone or in combination with saquinavir. The pathogenic mechanism has not been demonstrated from renal biopsies or experimental studies. Renal function should be followed in these patients and risk factors controlled.