Objectives. To assess the efficacy of a text messaging (SMS) strategy to improve appointment attendance, treatment adherence, and biological markers (viral load and CD4) in continuous patients with high activity antiretroviral therapy (HAART) who were late to their last scheduled appointment. Materials and Methods. A randomized controlled trial implemented in Via Libre, a non-governmental organization (NGO) that provides services to people living with human immunodeficiency virus (HIV) in Lima, Peru, was conducted, where 166 follow-up patients were randomized: 82 participants received SMS for six months and 84 participants received standard care. Results. Patients in the intervention group: 93.9% were men and had a median of 5.1 years in HAART; in the control group: 94.1% were men and a median of 5.3 years in HAART. In the intervention group, patients attended their scheduled medical appointments more frequently than those in the control group (RR=1.89, 95% CI 1.21-2.97) during the six months of intervention. Comparing the viral load level and CD4 level, no differences were found (p=0.930 and p=0.905, respectively). Adherence to treatment measured by self-report was higher in the intervention group (p<0.001). Conclusions. The results of this study suggest that sending SMS for six months may improve appointment attendance and adherence to treatment in continuing patients on HAART.