BACKGROUND & AIMS: Adherence to HCV therapy impacts sustained virological response (SVR) but there are limited data on adherence, particularly among injecting drug users (IDUs). We assessed 80/80 adherence (>/=80% of PEG-IFN doses, >/=80% treatment), on-treatment adherence, and treatment completion in a study of treatment of recent HCV infection (ATAHC). METHODS: Participants with HCV received pegylated interferon (PEG-IFN) alfa-2a (180mug/week, n=74) and those with HCV/HIV received PEG-IFN alfa-2a with ribavirin (n=35), for a planned 24 weeks. Logistic regression analyses were used to identify predictors of PEG-IFN 80/80 adherence. RESULTS: A total of 109 out of 163 patients received treatment (HCV, n=74; HCV/HIV, n=35), with 75% ever reporting IDU. The proportion with 80/80 PEG-IFN adherence was 82% (n=89). During treatment, 14% missed >/=1 dose (on-treatment adherence=99%). Completion of 0-4, 5-19, 20-23, and all 24 weeks of PEG-IFN therapy occurred in 10% (n=11), 14% (n=15), 6% (n=7) and 70% (n=76) of cases, respectively. Participants with no tertiary education were less likely to have 80/80 PEG-IFN adherence (AOR 0.29, p=0.045). IDU prior to or during treatment did not impact 80/80 PEG-IFN adherence. SVR was higher among those patients with >/=80/80 PEG-IFN adherence (67% vs. 35%, p=0.007), but similar among those with and without missed doses during therapy (73% vs. 60%, p=0.309). SVR in those patients discontinuing therapy between 0-4, 5-19, 20-23, and 24 weeks was 9%, 33%, 43%, and 76%, respectively (p<0.001). CONCLUSIONS: High adherence to treatment for recent HCV was observed, irrespective of IDU prior to, or during, therapy. Sub-optimal PEG-IFN exposure was mainly driven by early treatment discontinuation rather than missed doses during therapy.