Objectives. To measure the impact of different outreach messages on health insurance enrollment among Medicaid-eligible adults.Methods. Between March 2015 and April 2016, we conducted a series of experiments using mail-based outreach that encouraged individuals to enroll in Pennsylvania’s expanded Medicaid program. Recipients were randomized to receive 1 of 4 different messages describing the benefits of health insurance. The primary outcome was the response rate to each letter.Results. We mailed outreach letters to 32 993 adults in Philadelphia. Messages that emphasized the dental benefits of insurance were significantly more likely to result in a response than messages emphasizing the health benefits (odds ratio = 1.33; 95% confidence interval = 1.10, 1.61).Conclusions. Medicaid enrollment outreach messages that emphasized the dental benefits of insurance were more effective than those that emphasized the health-related benefits.Public Health Implications. Although the structure and eligibility of the Medicaid program are likely to change, testing and identifying successful outreach and enrollment strategies remains important. Outreach messages that emphasize dental benefits may be more effective at motivating enrollment among individuals of low socioeconomic status.