The primary purpose of this study is to evaluate the impact of changing didanosine in an effective anti-HIV regimen to tenofovir on virologic suppression. We hypothesize that, in patients with maximal virologic suppression on a double class regimen (including two NRTIs and an NNRTI or a PI, boosted with RTV or not), a single drug substitution of didanosine for tenofovir will represent a viable strategy without any negative impact on the virologic efficacy of the regimen.

Primary objective - to determine the impact of changing part of an effective HAART regimen to tenofovir on maintenance of virologic suppression in HCV co-infected patients.

Secondary objective - to assess the safety and tolerability over 12 weeks in patients switched to tenofovir.

Research Method - This will be a single arm observational study to include 30 subjects. Patients requiring HCV treatment will be assessed and patients receiving didanosine will be clinically evaluated to determine an appropriate NRTI drug switch. Patients who are to switch the didanosine component of their regimen to tenofovir will be eligible to participate in the study and will be followed for a period of observation of up to 4 weeks. All patients will be receiving tenofovir as one capsule, once daily. The primary endpoint will be maintenance of virologic suppression between the Baseline visit and week 12 in the overall study group. Measures of adherence to HAART, safety, tolerability and CD4 cell counts will also be obtained at each study visit, and will constitute secondary study endpoints.

Inclusion Criteria:

1. Be age 19 or older;
2. Have a confirmed diagnosis of HIV infection;
3. Have a confirmed positive HCV RNA PCR;
4. Have two consecutive HIV RNA levels <50 copies/mL with the most recent within the past 3 months;
5. Must not exhibit evidence of an acute illness, including an acute opportunistic infection;
6. Must not have any evidence of grade 3-4 laboratory abnormalities;
7. Must be able and willing to provide informed consent.

Exclusion Criteria:

1. Be receiving investigational drug within 30 days prior to beginning this study;
2. If female, be pregnant or breast-feeding;
3. In the opinion of the investigator, is unlikely to comply with the study protocol or is unsuitable for participation for any reason.