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Data from 197 patients for whom highly active antiretroviral therapy (HAART) failed, who started a new regimen chosen under the guide of resistance testing results interpreted by experts, were retrospectively studied, provided that at least 2 determinations of adherence and plasma drug concentrations were performed during the follow-up. Univariate and multivariable logistic regression analyses were conducted, using confirmed virologic response at week 24 as outcome measure (i.e., achievement of undetectable HIV plasma viral load at any time point before week 24 and its maintenance up to week 24). Suboptimal drug concentrations (odds ratio [OR]: 0.3; 95% confidence interval [CI] 0.2–0.7; p = 0.006) and suboptimal adherence (OR: 0.4; 95% CI 0.2–0.8; p = 0.014) were both negative independent predictors of sustained virologic response, while the use of boosted protease inhibitor-containing regimens resulted to be protective (OR: 2.4; 95% CI 1.1–5.3; p = 0.032).