Suppression of human immunodeficiency virus type 1 plasma virus load (PVL) to <20 copies/mL is associated with a longer virologic response after initiation of antiretroviral therapy. The relationship between duration of virologic response and PVL nadir according to a less sensitive assay was explored. When compared with subjects with a PVL nadir >500 copies/mL, the relative risks of PVL rising above 1000 copies/mL for participants in the INCAS trial and the British Columbia Drug Treatment Program with a PVL nadir below the limit of detection (LOD) were 0.04 (95% confidence interval [CI], 0.02–0.09) and 0.06 (95% CI, 0.03–0.12), respectively. The corresponding relative risks for persons with a detectable but not quantifiable PVL nadir were 0.25 (95% CI, 0.13–0.50) and 0.54 (95% CI, 0.25–1.19). The relative risks of virologic failure associated with a PVL nadir detectable but not quantifiable and a PVL nadir below the LOD were statistically different (P < .0001) in both data sets.