Patients are randomized to one of three treatment arms: AZT plus ddI, AZT plus ddC, and AZT alone alternating monthly with ddI. Half of the patients receiving AZT alternating monthly with ddI will start with AZT, while the other half will start with ddI. Treatment continues until death or termination of the study. Patients are followed every 4 weeks. The study will include a subset of patients for whom virologic, pharmacokinetic, and macroneurologic assessments will be made.