Human immunodeficiency virus (HIV) RNA was detected and quantified in the serum of HIVseropositive individuals using the polymerase chain reaction (PeR) and a nonisotopic enzymelinked affinity assay. Of 55 HIV-infected patients who were not receiving therapy, serum HIV RNA was detected in 9 of 19 who were asymptomatic, 11 of 16 with AIDS-related complex (ARC), and 18of 20 with AIDS, with copy numbers ranging from 102 to ⩾5 × 104/200 µl of serum based on a relationship between absorbance and known copy number of gag gene RNA. Linear regression analysis demonstrated a correlation between infectious titer in 42 patient sera cocultured with donor peripheral blood mononuclear cells (PBMC) and PCR product absorbance (r = .70, P > .01). Serum HIV RNA detected by PCR also correlated with serum p24 antigen positivity, CD4 counts 3, and the presence of HIV-related symptoms or disease. Quantification of infectious HIV RNA in cell-free serum by PCR may be useful as a marker for disease progression or in monitoring antiviral therapy.