OBJECTIVES: In the absence of reminders, multi-dose vaccine completion rates for adolescents and adults are low. Human papillomavirus (HPV) vaccine completion, for example, is 32% among US adolescents.  In the IMPACT trial (Improving Patient Adherence through Clinic-initiated Text messages), we examine the efficacy of automated text message-based reminders in improving series completion rates among patients ≥9 years old who have begun, but not completed, one or more multi-dose vaccine series: hepatitis B (HBV) or HPV. METHODS: In this multi-center randomized controlled trial, consenting participants (N=334) were assigned during a clinical visit to receive either 1) text message-based reminders before their next dose, or 2) traditional appointment reminders. In group 1, text messages were delivered to patients’ or parents' cellular telephones using a web-based application that automatically sends text reminders prior to appointments dates entered by clinical staff. We evaluated the association between treatment group and appointment attendance and vaccine series completion RESULTS: Among all participants, HPV or HBV series completion within 6 months of enrollment was higher for those receiving a text messages (39.8 % vs. 32.0%), but with meaningful effect modification by age. Among adult patients, those who received a text were more likely to complete their vaccine series for HPV or HBV (37.5% vs. 24.2%; risk ratio [RR] = 1.55). Among adult patients who initiated the HPV series alone, text messages doubled the rate of vaccine completion (37.5% vs. 18.5%; RR = 2.03). Text messaging was not effective for increasing series completion among adolescent participants. Text messaging improved overall appointment attendance at 3 of the 4 clinical sites, but the differences were not statistically significant. CONCLUSIONS: In this pilot study, text messaging initiated by the clinical office demonstrated improved multi-dose vaccine series completion among adult patients and showed promising trends for reducing appointment no-show rates.