Foscarnet (FC) is a new antiviral agent which has been recently proposed for the treatment of severe cytomegalovirus (CMV) infections in immunocompromised patients. When used intravenously (i.v.), main adverse effects of FC are a fall in hemoglobin, and an increase in liver enzymes and serum calcium. Although increased serum creatinine have been noted in several patients, deterioration of renal function is often accounted for by the concomitant use of other nephrotoxic drugs, the severity of underlying disease or the presence of graft rejection. Consequently FC is often considered as a non or poorly nephrotoxic drug. We report 4 cases of acute renal failure (ARF) which can be exclusively attributed to FC. FC was used for CMV chorioretinitis in 3 AIDS patients and in one non-immunocompromised patient. ARF was diagnosed between the 6th and 15th day of treatment, with oligoanuria in two patients (one of whom required two hemodialysis periods). ARF was most likely secondary to acute toxic tubulopathy. Three patients did not receive any other nephrotoxic drug. The fourth patient received concomitantly sulfadiazine but renal function returned to baseline value after FC completion although sulfadiazine was continued. In conclusion, our 4 observations suggest that FC may be responsible for acute tubulopathy. We suggest that in these patients renal function should be carefully monitored and dehydration promptly corrected to limit the risk of nephrotoxicity.