To elucidate the relationship between virologic risk factors and immunologic and clinical progression in patients receiving monotherapy in protocol ACTG 175, and to compare new treatment regimens with combinations of reverse transcriptase inhibitors in long-term recipients of monotherapy. Specifically, to determine, in patients who have been taking zidovudine (AZT) alone for a long time, whether it is beneficial to add lamivudine (3TC) to AZT or to switch to d4T alone, and also to determine, in patients who have been taking didanosine (ddI) alone for a long time, whether it is beneficial to add AZT or AZT/3TC to ddI.