Background: Text messaging (SMS) interventions to improve HIV retention in care have shown mixed results, probably due to the diversity of features of their design and implementation. Methods: Using routinely collected data of the largest HIV program in Lima, we retrospectively evaluated the efficacy of an adapted single-arm, one-way SMS pilot intervention, intended to improve compliance with medical and ART pick-up appointments, by use of (i) SMS reminders before scheduled visits, (ii) a motivational SMS after scheduled visit, and (iii) phone calls after missed visits delivered for up to 9 months. The SMS group included people living with HIV (PLWH) ages 18-29 starting or restarting antiretroviral therapy (ART). For the control group, we non-randomly selected PLWH up to 35 years-old who were enrolled in the same HIV program within overlapping or closest dates to those of the SMS group (...