Serial samples from source plasma donors with confirmed new HIV infection were investigated for low-level viremia (LLV) (ie, < 100 genome copies [cp]/mL) at time points preceding the period of steadily rising viremia above 100 cp/mL (ramp-up viremia). Fifteen of 44 plasma donor panels previously studied for the dynamics of HIV viremia during primary infection contained 70 samples with undetectable HIV-1 RNA by quantitative polymerase chain reaction (PCR). On retesting with a sensitive qualitative reverse transcriptase PCR assay (95% detection at 4 cp/mL), we identified LLV in 13 of 15 panels and 23 of 69 retested samples. In 6 panels, a total of 11 samples (1-3 per panel) were consistent with LLV before ramp-up viremia. These samples preceded the first sample with >100 cp/mL HIV by 9 to 25 days (median = 18 days) and were separated from the latter by at least 1 sample with undetectable viremia by the qualitative PCR assay. We conclude that LLV is not uncommon during the very early period of primary HIV infection preceding ramp-up viremia. It is not known if blood is infectious during this period; however, given the low viral concentrations and transient nature of the observed viremic "blips," the risk of infectivity can be assumed to be small.