The currently available data on clinical events in patients receiving potent antiretroviral therapy suggest that an alteration in the presentation of MAC disease may be seen and that rates of MAC disease may be reduced when patients respond to antiretroviral therapy. However, the extent of the protection and the timing of protection after initiation of therapy remain unknown. The current study should provide validated measures of immune restoration and clinical data to guide prophylaxis decisions for the many patients who are now responding to therapy after years of immune depletion. [AS PER AMENDMENT 11/16/99: The low rate of MAC in ACTG 362 patients after an average of 1 year of follow-up suggests that prophylaxis specifically for MAC disease with azithromycin is not necessary for patients who have experienced immune reconstitution. Prolonged follow-up will define durability of the antiretroviral response and the experience with opportunistic conditions, neurologic diseases, and survival, especially in those whose CD4 counts drop below 50 cells/mm3. It will also allow assessment of the levels of CD4 cell number at which vulnerability to opportunistic infection recur.] [AS PER AMENDMENT 03/18/03: During the extension of ACTG 362, serious complications of HAART have become better defined, including metabolic complications, neurologic problems, atypical opportunistic infections, and malignancies. Patients in ACTG 362 have been exposed to HAART longer than any other large group in the ACTG, and appear to benefit from and suffer complications of their therapy. Continued observation should provide estimates of expected complications and durability of long-term potent antiretroviral treatment, and may detect unanticipated problems.]