There are 3 groups in this study. Group 1 will comprise HIV infected participants currently taking a PI-containing regimen. Group 2 will comprise HIV infected participants who are not taking PIs. Group 3 will comprise HIV uninfected participants. At each site, participants will be enrolled by "triads" consisting of an individual from each group. Each member of the triad will be closely matched by gender, age, smoking and menopausal status, race, and normal or hypertensive blood pressure. All 3 members of the triad must be identified before participants are registered to the study.