Background: Sedentary behaviour (sitting) is a highly prevalent negative health behaviour, with individuals of all ages exposed to environments that promote prolonged sitting. The sedentary school classroom represents an ideal setting for environmental change, through the provision of sit-stand desks. Objectives: The aim of this study was to undertake a pilot cluster RCT of the introduction of sit-stand desks in primary school classrooms to inform a definitive trial. Objectives included providing information on school and participant recruitment and retention, acceptability of the intervention and outcome measures. A preliminary estimate of intervention effectiveness on the proposed primary outcome (change in weekday sitting time) for inclusion in a definitive trial was calculated, along with a preliminary assessment of potential cost-effectiveness. A full process evaluation was also undertaken. Design: A two-armed pilot cluster RCT with economic and qualitative evaluations. Schools were randomised on a 1:1 basis to the intervention (n=4) or control (n=4) arms. Setting: Primary schools in Bradford, West Yorkshire, UK Participants: Children in Year 5 (aged 9-10 years). Intervention: Six sit-stand desks replaced three standard desks (sitting 6 children) in the intervention classrooms for 4.5-months. Teachers were encouraged to ensure all pupils were exposed to the sit-stand desks for at least 1 hour/day on average using a rotation system. Schools assigned to the control arm continued with their usual practice. Main outcome measures: Trial feasibility outcomes included school and participant recruitment and attrition, acceptability of the intervention, and acceptability and compliance to the proposed outcome measures (including activPAL-measured weekday sitting, physical activity, adiposity, blood pressure, cognitive function, musculoskeletal comfort, academic progress, engagement and behaviour). Results: 33% of schools approached and 75% (n=176) of eligible children took part. At 7-months follow-up, retention rates were 100% for schools and 97% for children. Outcome measure completion rates ranged from 63–97%. A preliminary estimate of intervention effectiveness, from a weighted linear regression model (adjusting for baseline sitting time and wear time) revealed a mean difference in change in sitting of -30.6 minutes/day (95% CI: -56.42 to -4.84) between intervention and control groups. The process evaluation revealed the intervention, recruitment and evaluation procedures were acceptable to teachers and children, with the exception of minor issues around activPAL attachment. A preliminary within-trial economic analysis revealed no difference between intervention and control groups in health and education resource use or outcomes. Long-term modelling estimated an unadjusted incremental cost-effectiveness ratio of ‘Stand Out in Class’ of £78,986 per QALY gained. Limitations: The intervention duration (4.5 months) was shorter than originally planned. Conclusion: This study has provided evidence of the acceptability and feasibility of the Stand Out in Class intervention and evaluation methods. Preliminary evidence suggests the intervention may have a positive direction of effect on weekday sitting time which warrants testing in a full cluster RCT. Future work: Lessons learnt from this trial will inform the planning of a definitive trial. Study registration: ISRCTN12915848 (retrospectively registered, date: 09/11/16) Funding details: This study was funded by the National Institute for Health Research Public Health Research Programme (reference: 14/231/20).