Background Socioeconomic status (SES) is associated with asthma morbidity in observational studies, but the factors underlying this association are uncertain. Objective We investigated whether 3 SES correlates—low income, low education, and high perceived stress—were independent risk factors for treatment failure and asthma exacerbations in the context of a randomized controlled trial. Methods The effect of low SES (household income of <$50,000/y and household educational level of less than a Bachelor's degree) and high perceived stress (defined as a score of >20 on a perceived stress scale) on asthma morbidity was analyzed in 381 participants by using Poisson regression models. The primary outcome was treatment failure (defined in the trial protocol as a significant clinical or airflow deterioration), and the secondary outcome was asthma exacerbations requiring systemic corticosteroids. Results Fifty-four percent of participants had a low income, 40% had a low educational level, and 17% had high perceived stress levels. Even after adjusting for race and other important confounders, participants with lower income had higher rates of both treatment failures (rate ratio, 1.6; 95% CI, 1.1-2.3; P  = .03) and exacerbations (rate ratio, 1.9; 95% CI, 1.1-3.3; P  = .02). Adherence with inhaled corticosteroids was similarly high for both income categories. Education and perceived stress were not significantly associated with either outcome. Conclusions In the context of a randomized controlled trial, participants with lower income were more likely to experience adverse asthma outcomes independent of education, perceived stress, race, and medication adherence.