ABSTRACT: Summary: Background:Antiretroviral therapy (ART) non-adherence causes HIV treatment failure. Past behaviour might predict future behaviour; failing second-line ART could indicate ongoing risk for subsequent non-adherence. We aimed to find out whether a two-way mobile phone-based communication intervention would increase HIV treatment success by improving medication adherence. Methods:We did a multinational, randomised controlled trial of patients at 17 sites in nine lower-income and middle-income countries in Africa, Asia, and the Americas. Patients aged 18 years and older, with HIV infection, and on second-line protease-inhibitor-based antiretroviral regimens, were randomly assigned (1:1) to either two-way mobile phone intervention plus standard of care (MPI + SOC) adherence support or standard-of-care alone (SOC). Our study was nested within a strategy study of ART after second-line ART failure (the main study, A5288). The main study had four cohorts, which were assigned regimens according to ART history and real-time genotype. Randomisation was stratified by the main study cohort with dynamic institutional balancing. Only the clinical management committee was masked, not the participants or site personnel. Text messages were sent over 48 weeks starting once a day and tapering down to once per week; participants were to respond once to each message if taking ART without issues. Repeated non-response to three messages over a 2-week period for the first 8 weeks, and then two messages over a 2-week period for the remainder of the study, triggered problem-solving counselling by staff. For this study, the primary endpoint was plasma HIV-1 RNA 200 copies per mL or less at 48 weeks and the secondary endpoint was virological failure (two consecutive HIV-1 RNA ?1000 copies per mL) at 24 or more weeks. Prespecified intention-to- treat analyses were adjusted for cohort. Follow-up continued until the last participant had reached 48 weeks, with a median follow-up time of 72 weeks. The trial is registered with ClinicalTrials.gov, number . Findings:Enrolment began on Feb 22, 2013, and ended on Dec 21, 2015, with the last participant completing follow-up on Feb 13, 2017. Of 545 participants in the main study, 521 (96%) were enrolled and randomly assigned to MPI + SOC (n=257) or SOC alone (n=264). 52% of patients were men and the median HIV-1 RNA 4-4 log10 copies per mL (IQR 3.5 to 5-2). At week 48, HIV-1 RNA 200 copies per mL or less was reached in 169 (66%) of 257 patients in the MPI + SOC group and 164 (62%) of 264 patients in the SOC group (estimated difference 3-6% [95% CI -4-6% to 11-9%]; p=0-39). The adjusted odds ratio comparing MPI + SOC and SOC was 1-23 (0-82 to 1-84; p=0-32). Virological failure occurred in 66 (26%) patients in the MPI + SOC group and 89 (34%) patients in the SOC group during the median 72 weeks follow-up (adjusted p=0-027). Observed difference in virological failure favoured MPI + SOC in all cohorts. 23 (4%) participants died, 11 (4%) in the MPI + SOC group and 12 (5%) in the SOC group (p=0-89), with none of the deaths ascribed to ART, the MPI, or study procedures. Interpretation:Two-way MPI did not significantly improve week 48 suppression, but it did modestly affect virological failure. People failing second-line ART might not achieve benefits from phone-based triggers or enhanced adherence support (or both). More effective strategies are needed.