['The contribution of low frequency drug-resistant human immunodeficiency virus type 1 (HIV-1) variants to failure of antiretroviral therapy is not well defined in treatment-experienced patients. We sought to detect minor nonnucleoside reverse-transcriptase inhibitor (NNRTI)-resistant variants at the initiation of multidrug efavirenz-containing therapy in both NNRTI-naive and NNRTI-experienced patients and to determine their association with virologic response.', 'Plasma samples at entry and at time of virologic failure from patients enrolled in the AIDS Clinical Trials Group study 398 were analyzed by standard genotype, single-genome sequencing and allele-specific polymerase chain reaction (K103N and Y181C) to detect and quantify minor NNRTI-resistant variants.', 'Minor populations of NNRTI-resistant variants that were missed by standard genotype were detected more often at study entry in NNRTI-experienced patients than NNRTI-naive patients by both single-genome sequencing (8 of 12 vs 3 of 15; P = .022) and allele-specific polymerase chain reaction (11% Y181C, 5 of 22 vs 3 of 72, respectively; P = .016). K103N variants at frequencies 11% were associated with inferior HIV-1 RNA response to efavirenz-containing therapy between entry and week 24 (change in HIV-1 RNA level, +0.5 vs -1.1 log(10) copies/mL; P < .001).'] Do low frequency nonnucleoside reverse-transcriptase inhibitor-resistant variants contribute to failure of efavirenz-containing regimens in treatment- experienced patients?