Virological failure of first-generation nonnucleoside reverse transcriptase inhibitors (NNRTIs) can compromise the efficacy of etravirine as a result of the accumulation of NNRTI resistance mutations. How quickly NNRTI resistance accumulates in patients with a delayed switch from nevirapine or efavirenz despite virological failure, when these drugs are used as a component of combination antiretroviral therapy (cART), remains unclear.