One hundred-twenty patients who have been receiving AZT for at least 1 year are randomized to 1) continue with AZT, 2) switch to treatment with didanosine at 1 of 2 doses, or 3) receive both AZT and ddI. Treatment is given for 16 weeks, with a possible extension to 32 weeks. Patients are followed at weeks 2, 4, 8, 12, and 16. For analysis purposes only, patients are stratified according to degree of susceptibility of HIV isolates to AZT.