From a prospective cohortstudy,24 asymptomatic men were identified who had been antibody positive for human immunodeficiency virus (HIV) for at least 5 years (median = 9.1) with CD4+ lymphocyte counts ⩾400 cells/mm3, Of these “nonprogressors,” 23 (96%) had evidence of HIV infection by either HIV culture or the polymerase chain reaction (PCR) for HIV DNA, although only 1 (4%) had a positive assay for HIV RNA (by PCR)and no one was positive for p24 antigen. Compared with 24 antibody-negative men and 14 men with AIDS, nonprogressors had higher CD8+ counts and lower natural killer cell activity. Nonprogressors had higher β2-microglobulin levels than did seronegative controls, suggesting somedegree of immune system activation. Compared with men with AIDS, nonprogressors seemed to have a stronger antibody response to six different HIV-related proteins but did not differ significantly in neutralizing antibody or antibody dependent cellular cytotoxic activity.