HIV-infected patients 18 years of age and older may be eligible for this study. Patients involved in the viral load test will be recruited from an NIAID HIV study in which they are already participating. Three groups of patients will be enrolled: those with a viral load of less than 50 copies/ml plasma, those with 51-500 copies/ml, and those with 501-5000 copies/ml. Patients involved in the Kaletra trial must have been taking HAART for 6 months or more and have less than 50 viral copies/ml plasma. They will be screened for this study with a history, physical examination, and routine laboratory tests.