Two dose levels of ddI, each adjusted depending on patient's weight at study entry, are compared with a variable dosage regimen of AZT (the dose which the patient is tolerating at the time of study entry). Randomization is stratified by baseline CD4 cell count (less than 100 or 100-300) and Medical Center. This study continues for at least 12 months after the entry of the first subject. Patients randomized to AZT will receive orally. All patients randomized to AZT also receive a ddI placebo at 12 hour intervals. Patients randomized to ddI receive AZT placebo.