Patients 18 years of age or older with HIV-1 infection who have been receiving highly active antiretroviral therapy (HAART) may be eligible for this study. In order to study the effect of increased levels of virus on the immune system, therapy will be stopped in these patients temporarily. Therefore, only patients who have an appropriate level of understanding of the potential benefits of therapy and the risks of stopping treatment will be considered for enrollment. Pregnant women may not participate and women of childbearing potential must agree not to become pregnant during the study. Candidates will be screened with a medical history, physical exam, blood and urine tests and possibly a chest X-ray and electrocardiogram.