The aim of this trial is to study the efavirenz-rifabutin interaction. Thus, this trial will enroll patients with HIV and tuberculosis co-infections who are receiving a rifabutin-based regimen and who plan to begin an antiretroviral regimen containing efavirenz dosed at 600 mg daily. Enrollment in TB Trials Consortium Study 23 is not a requirement for participation in this study.

Primary Objective:

To compare the pharmacokinetics of rifabutin at 600 mg twice a week in combination with efavirenz 600 mg daily to the pharmacokinetics of rifabutin 300 mg twice a week without efavirenz.

• Patients with HIV and tuberculosis co-infections who are receiving a rifabutin-based regimen and who plan to begin an antiretroviral regimen containing efavirenz dosed at 600 mg daily.