Additional samples will then be collected as often as daily for the first week, twice a week for the next 3 weeks and then weekly to monthly for up to 1 year. Some patients may undergo a tissue biopsy (removal of a small tissue sample from a lymph node, tonsil or colon) or computed tomography (CT) scans of the thymus (a small gland between the lungs that manufactures lymphocytes. Some patients will have a second infusion in order to examine changes in the rate of CD4 replication over time or following potent antiretroviral therapy. Patients will be followed in the clinic periodically for the first year and then will be seen in the clinic or contacted by telephone once a year for 4 more years.