The study sites will provide ABC, 3TC, ddI, or d4T to all patients who are assigned to take these medications. All other anti-HIV drugs for initial and subsequent treatment regimens are obtained by clinician prescription. At Months 1 and 4 and then every 4 months thereafter, patients will receive a medical history update, physical exam, and questionnaire. Blood samples will also be drawn to measure CD4 cell count, viral load, and genotypic antiretroviral resistance. Changes in treatment regimens may occur at any time.