Abstract Background Xentuzumab—a humanised IgG1 monoclonal antibody—binds IGF-1 and IGF-2, inhibiting their growth-promoting signalling and suppressing AKT activation by everolimus. This phase Ib/II exploratory trial evaluated xentuzumab plus everolimus and exemestane in hormone receptor-positive, locally advanced and/or metastatic breast cancer (LA/MBC). Methods Patients with hormone receptor-positive/HER2-negative LA/MBC resistant to non-steroidal aromatase inhibitors were enrolled. Maximum tolerated dose (MTD) and recommended phase II dose (RP2D) of xentuzumab/everolimus/exemestane were determined in phase I (single-arm, dose-escalation). In phase II (open-label), patients were randomised 1:1 to the RP2D of xentuzumab/everolimus/exemestane or everolimus/exemestane alone. Randomisation was stratified by the presence of visceral metastases. Primary endpoint was progression-free survival (PFS). Results MTD was determined as xentuzumab 1000 mg weekly plus everolimus 10 mg/day and exemestane 25 mg/day. A total of 140 patients were enrolled in phase II (70 to each arm). Further recruitment was stopped following an unfavourable benefit-risk assessment by the internal Data Monitoring Committee appointed by the sponsor. Xentuzumab was discontinued; patients could receive everolimus/exemestane if clinically indicated. Median PFS was 7.3 months (95% CI 3.3–not calculable) in the xentuzumab/everolimus/exemestane group and 5.6 months (3.7–9.1) in the everolimus/exemestane group (hazard ratio 0.97, 95% CI 0.57–1.65; P = 0.9057). In a pre-specified subgroup of patients without visceral metastases at screening, xentuzumab/everolimus/exemestane showed evidence of PFS benefit versus everolimus/exemestane (hazard ratio 0.21 [0.05–0.98]; P = 0.0293). Most common any-cause adverse events in phase II were diarrhoea (29 [41.4%] in the xentuzumab/everolimus/exemestane group versus 20 [29.0%] in the everolimus/exemestane group), mucosal inflammation (27 [38.6%] versus 21 [30.4%]), stomatitis (24 [34.3%] versus 24 [34.8%]), and asthenia (21 [30.0%] versus 24 [34.8%]). Conclusions Addition of xentuzumab to everolimus/exemestane did not improve PFS in the overall population, leading to early discontinuation of the trial. Evidence of PFS benefit was observed in patients without visceral metastases when treated with xentuzumab/everolimus/exemestane, leading to initiation of the phase II XENERA™-1 trial (NCT03659136). Trial registration ClinicalTrials.gov, NCT02123823 . Prospectively registered, 8 March 2013.