HIV-infected patients will have a MRI scan of the hip bone, and possibly other bones. Before the scan, they will provide a medical history by interview or questionnaire and have a physical examination. Those with diagnosed AVN will also have blood drawn to help identify potential risk factors for AVN. For the MRI, the patient lies flat on a stretcher that is moved into a cylindrical machine with a strong magnetic field. Earplugs are worn to muffle thumping sounds produced by radio waves that form the images. Patients can speak with a staff member via an intercom system at all times during the procedure. Repeat MRI studies will be requested at approximately 3- to 12-month intervals for up to 5 years, to see if changes occur with time. If medically indicated, additional diagnostic studies may be done to obtain information needed for appropriate medical care. Blood will be drawn periodically to examine immune parameters and viral blood levels. Patients with AVN may have periodic consultations with the rehabilitation medicine service and orthopedic surgeons.