We evaluated the safety and efficacy of peginterferon alpha-2a (pegIFNalpha-2a), with or without ribavirin, in 154 HCV/HIV coinfected patients. All received pegIFNalpha-2a (180 microg/week) for 12 weeks, with those achieving an early virologic response (EVR) continued on monotherapy through week 48. Patients without an EVR were randomized at week 14 to also receive ribavirin (800 mg/day) or placebo through week 48. Patients with detectable HCV RNA at week 24 were discontinued. An EVR occurred in 59 of 154 patients on monotherapy, and a sustained virologic response (SVR) occurred in 19 of 55 of those achieving an EVR and continuing monotherapy through week 48. One week 12 nonresponder receiving pegIFNalpha-2a plus ribavirin, and none receiving pegIFNalpha-2a plus placebo, achieved a SVR. Discontinuations for adverse events occurred in 10 of 154 patients before, and 16 of 131 after, week 14. HIV RNA and CD4 counts did not change significantly during treatment. PegIFNalpha-2a was therefore at least as effective as standard interferon and ribavirin combination therapy and was well tolerated, without a negative impact on HIV parameters.