Cidofovir, (S)-1-[3-hydroxy-2-(phosphonylmethoxy)propyl]cytosine, is a novel antiviral nucleotide analogue with potent in vitro and in vivo activity against cytomegalovirus (CMV) and other herpesviruses. Thirty-one human immunodeficiency virus-seropositive patients with asymptomatic CMV excretion were evaluated in a phase I/II study with 2 regimens of cidofovir: cidofovir alone at doses of 0.5, 1.0, 3.0, or 10.0 mg/kg/week (20 patients) and cidofovir at 3.0, 5.0, or 7.5 mg/kg with concomitant oral probenecid, saline prehydration, extended dosing intervals, and drug interruption for proteinuria (19 patients). Prolonged and dose-dependent anti-CMV effect was observed with all cidofovir regimens > or = 3.0 mg/kg. The dose-limiting toxicity of cidofovir was dose- and schedule-dependent nephrotoxicity. Four of 20 patients had serum creatinine levels > or = 2.0 mg/dL after a mean cumulative exposure of 14.8 mg/kg cidofovir alone; however, none of 19 patients receiving the modified regimen had elevated creatinine (mean cidofovir exposure, 32.2 mg/kg). The clinical efficacy of cidofovir and its potential for cumulative nephrotoxicity needs further study in patients with end-organ CMV disease.