The cohort consisted of 3 groups: Group A HIV seropositive, no symptoms attributable to HIV and CD4+ Cells >= 400 per microliter; Group B HIV seropositive chemical manifestations of HIV in past 6 months or CD4+ Cells < 400 per microliter; and Group C HIV seronegative controls. The pulmonary status of individuals in each of the categories was evaluated by such methods as chest radiography, pulmonary function tests, nuclear medicine studies, and histological and/or microbiological evaluation. The prospective cohort study described the incidence and course of lung diseases at all stages of HIV infection. Six clinical centers from different geographic areas in the United States began enrolling participants in 1988, and the resulting cohort comprised 1,369 members. HIV seropositive participants were randomized to "intensive" (pulmonary disease screening and follow-up at three-month intervals) or "routine" (six-month follow-up intervals with annual screening) follow-up to assess the impact of these strategies on patient outcomes. The contract-supported phase of the study was jointly funded by the NHLBI and the NIAID.