Mononuclear cells were obtained from 71 human immunodeficiency virus type 1 (HIV-1) seropositive subjects presenting and first visit either as asymptomatic or with minor symptoms and with CD4 lymphocytes greater than 550 per mm3 (group A, 35 patients) or as patients with AIDS, AIDS-related illnesses, or CD4 lymphocytes less than 400 per mm3 (group B, 36 patients). After 1-5 years of follow-up, 13 patients of group A had essentially retained their initial status (asymptomatics); the 22 others had suffered clinical or immunological deterioration (progressors). Frozen cells were thawed and submitted to lethal gamma-irradiation in vitro (4500 rads; 1 rad = 0.01 Gy) before they were cultured with normal phytohemagglutinin-stimulated lymphocytes to determine radiation-resistant HIV expression ex vivo (R-HEV). HIV antigenemia correlated with R-HEV values in 142 samples (r = 0.92, P less than 0.001) but was a less sensitive predictor of disease than R-HEV. R-HEV was detected in all specimens from patients with major AIDS-related illnesses or HIV-associated CD4 lymphopenia. In 77% of the progressors from group A, R-HEV detection preceded the onset of AIDS-associated disease or CD4 lymphopenia by 1 year (average). Conversely, R-HEV was low or was not detected in 36 sequential specimens from the 13 patients who remained asymptomatic over the following 2-5 years. Thus, persistently low HIV expression in vivo predicted a nondiseased state, whereas higher HIV expression levels seemed necessary for disease to occur. These data indicate that R-HEV is related to productive HIV infection in vivo, the latter acting as a determinant of AIDS-related illnesses. In view of this, measurement of HIV expression levels in the patient should be useful in antiviral efficacy trials.