Sitting time is a prevalent health risk among office-based workers.To examine, using a pilot study, the efficacy of an intervention to reduce office workers' sitting time.Quasi-experimental design with intervention-group participants recruited from a single workplace that was physically separate from the workplaces of comparison-group participants.Office workers (Intervention, n=18; Comparison, n=14) aged 20-65 years from Brisbane, Australia; data were collected and analyzed in 2011.Installation of a commercially available sit-stand workstation.Changes from baseline at 1-week and 3-month follow-up in time spent sitting, standing, and stepping at the workplace and during all waking time (activPAL3 activity monitor, 7-day observation). Fasting total cholesterol, high-density lipoprotein (HDL) cholesterol, triglycerides, and glucose levels were assessed at baseline and 3 months (Cholestech LDX Analyzer). Acceptability was assessed with a 5-point response scale (eight items).The intervention group (relative to the comparison group) reduced sitting time at 1-week follow-up by 143 minutes/day at the workplace (95% CI= -184, -102) and 97 minutes/day during all waking time (95% CI= -144, -50). These effects were maintained at 3 months (-137 minutes/day and -78 minutes/day, respectively). Sitting was almost exclusively replaced by standing, with minimal changes to stepping time. Relative to the comparison group, the intervention group increased HDL cholesterol by an average of 0.26 mmol/L (95% CI=0.10, 0.42). Other biomarker differences were not significant. There was strong acceptability and preference for using the workstations, though some design limitations were noted.This trial is the first with objective measurement and a comparison group to demonstrate that the introduction of a sit-stand workstation can substantially reduce office workers' sitting time both at the workplace and overall throughout the week.