Background: Acute rehabilitation in critically ill patients can improve post-ICU physical function. The feasibility and effect of in-bed cycling early in a patient's ICU stay is uncertain. Methods: We conducted a pilot randomized clinical trial (RCT) in 7 Canadian medical-surgical ICUs We included patients who could ambulate independently before ICU admission, within the first 4 days of invasive mechanical ventilation (MV) and first 7 days of ICU admission. Following informed consent, patients underwent concealed randomization to either 30 minutes/day of in-bed cycling and routine physiotherapy (Cycling) or routine physiotherapy alone (Routine) for 5 days/week, until ICU discharge. Feasibility outcome targets included: Accrual of 1-2 patients/month/site; >80% cycling protocol delivery; >80% outcomes measured; and >80% blinded outcome measures at hospital discharge. We report ascertainment rates for our primary outcome for the main trial (Physical Function ICU Test-scored [PFIT-s] at hospital discharge). Findings: Between 3/2015 and 6/2016, we randomized 66 patients (36 Cycling, 30 Routine). Our consent rate was 84.6% (66/78). Patient accrual was [mean (standard deviation)] 1.1(0.3) patients/month/site. Cycling occurred in 79.3%(146/184) of eligible sessions, with a median (IQR) session duration of 30.5(30.0, 30.7) minutes. We recorded 43(97.7%) PFIT-s scores at hospital discharge and 37(86.0%) of these assessments were blinded. Interpretation: Our pilot RCT suggests that a future multicenter RCT of early in-bed cycling for MV patients in the ICU is feasible. Funding: Canadian Frailty Network, Canadian Respiratory Research Network, Ontario Thoracic Society, Canadian Institutes of Health Research, Canada Foundation for Innovation, Ontario Ministry of Research and Innovation, Canada Research Chairs. Clinical Registry Number: Clinicaltrials.gov: NCT02377830. Protocol: BMJ Open.2016.6:e011659. Conflict of Interest Statement: This work was supported by grants from Technology Evaluation in the Elderly Network Catalyst (now Canadian Frailty Network; CAT2014-05), Canadian Respiratory Research Network Emerging Research Leaders Initiative, Ontario Thoracic Society Grant-in-Aid, and Canadian Institutes of Health Research Transitional Operating Grant (Award #142327), Canada Foundation for Innovation, and the Ontario Ministry of Research and Innovation. Michelle Kho and Deborah Cook are each funded by a Canada Research Chair. Restorative Therapies (Baltimore, MD) provided 2 RT-300 supine cycle ergometers for Toronto General Hospital and London Health Sciences sites for this research. The funding sources and equipment manufacturer had no role in the design, execution, analysis, or interpretation of this study, or decision to submit results. Ethics Approval Statement: Local ethics committees approved our research, which was conducted in accordance with the Declaration of Helsinki and national laws.