['To investigate the relation between the quantity of human immunodeficiency virus type 1 (HIV-1) RNA in plasma and the risk for the acquired immunodeficiency syndrome (AIDS) or a decline in the CD4+ T-cell count after seroconversion.', 'Prospective study.', '62 homosexual men with documented HIV-1 seroconversion.', 'University outpatient setting.', 'Clinical status, CD4+ T-cell counts, and plasma and serum samples were obtained every 6 months. Human immunodeficiency virus RNA in plasma was quantitated with a branched-DNA (bDNA) assay. Serum samples were assayed for neopterin, beta 2-microglobulin, and immune complex dissociated HIV-1 p24 antigen.', '18 of 62 (29%) men developed AIDS; 21 (34%) had a significant decline in the CD4+ T-cell count without AIDS; and 23 (37%) had a stable CD4+ T-cell count. For each participant, HIV-1 RNA results were categorized into one of four groups: 1) detection of HIV-1 RNA (> 1 x 10(4) genome equivalents/mL [Eq/mL]) in all samples; 2) detection in most samples (> or = 50%); 3) detection in fewer than 50% of samples; and 4) detection in none of the samples. Detection of HIV-1 RNA in all or most samples was strongly associated with AIDS (16 of 18 patients) and a decline in the CD4+ T-cell count (13 of 21 patients) compared with a stable CD4+ T-cell count (4 of 23 patients; P < 0.001). Conversely, the absence of HIV-1 RNA (< 1 x 10(4) Eq/mL) in all or most samples was associated with stable CD4+ T-cell counts (19 of 23 patients) and a lower risk for AIDS or decline in the CD4+ T-cell count (10 of 39 patients; P < 0.001). In multivariate analysis of all laboratory values at the seroconversion visit, a plasma HIV-1 RNA level greater than 1 x 10(5) Eq/mL was the most powerful predictor of AIDS (odds ratio, 10.8; P = 0.01).'] Does quantitation of HIV-1 RNA in plasma predict outcome after seroconversion?