Background: Ataxia-telangiectasia (A-T) is a rare multisystem disease characterised by neurodegenerative cerebellar ataxia, lung disease, immune deficiency, high cancer risk, and mitochondrial dysfunction. A-T cells demonstrate defective endoplasmic reticulum-mitochondrial connectivity disrupting calcium homoeostasis and mitochondrial fusion, which are corrected in vitro by the triheptanoin metabolite, heptanoate. Methods: We performed a Phase 2a/b trial of triheptanoin with a three-arm placebo-controlled dose-escalation design. Doses escalated at 2-month intervals for 12 months in the sequence 0%, 10%, 20%, 35% of calculated caloric intake. The primary outcome was cell death in respiratory epithelial cells. Key secondary outcomes included scales for assessment and rating of ataxia (SARA), international cooperative ataxia rating scale (ICARS), speech and swallowing function, and novel biomarker discovery. Findings: 31 participants with A-T were enrolled aged from 4 to 37 years (median 16-years). For the maximum dose vs. placebo or no dose, significant improvements was observed for the primary outcome percent nasal cell death (mean difference (MD) = −9.7%, 95% confidence interval (CI) −16.0, 4.6). The SARA subscale kinetic function improved (MD = −5.8, 95% CI −10.4, −1.2), as did ICARS subscales gait (MD = −0.5, 95% CI −0.9, −0.1) and fine motor disturbance (MD = −2.7, 95% CI −4.3, −1.1). Speech intelligibility (MD = −12.8, 95% CI −21.2, −4.3) and swallowing safety (−0.9, 95% CI −1.6, −0.3) improved. Adverse events including abdominal pain, nausea, vomiting, and diarrhoea, requiring dose capping at 20%, were observed in 12 (38%) participants. Interpretation: Improvements in mitochondrial function in A-T cells in vivo in patients occurred after triheptanoin. The biomarkers neurofilament light chain and interferon signature stimulated gene scores may allow for monitoring of disease progression and treatment response. Funding: Funded by Medical Researcher Futures Fund Australia (GA89314), The University of Queensland, Wesley Research Institute, and BrAshA-T.