The contribution of E138 mutations to etravirine resistance was investigated. Amino acids at position E138 after failure with etravirine in DUET were A (n = 1), G (n = 5), K (n = 3), P (n = 1), Q (n = 5), and V (n = 2). At baseline, only E138A and Q were found at 3.0% and 2.5%, respectively. Virologic response (less than 50 copies/mL) was observed in six of 12 and eight of 10 patients with E138A and E138Q, respectively. Site-directed mutants harboring E138A/G/K/Q/R or S showed etravirine fold change values of 2.9, 2.4, 2.6, 3.0, 3.6, and 2.8, respectively. E138G, K, and Q were added to the existing etravirine-weighted genotypic score including 17 etravirine resistance-associated mutations.