Objective: To evaluate whether prolonged operative time is negatively associated with post-operative complications and length of stay in patients undergoing microvascular free flap reconstruction for complex head and neck defects. Methods: 342 consecutive patients undergoing microvascular reconstruction for head and neck defects between 2017-2019 at a single institution were evaluated. Operative outcomes and operative time were compared whilst controlling for patient and treatment related factors. Results: Mean operative time was 551 minutes and length of stay was 16.2 days. An 11% increase in the risk of a post-operative complication was observed for every additional hour of operative time (OR 1.11, 95%CI 1.03 – 1.21, p=0.011) after adjusting for patient and treatment factors. A cut-off of 9 hours yielded a 92% increase in complications on either side of this (OR 1.92, 95%CI 1.18 – 3.13, p=0.009). Increased operative time was also associated with increased length of stay and return to theatres, but not medical complications. Conclusion: Prolonged operative time is significantly associated with increase surgical complications, length of stay and return to theatres when performing microvascular reconstructive surgery for head and neck defects. Keywords : free flap, microsurgery, operative time, length of hospital stay, complication rate, head and neck surgery Key points : 1. This study analyzed a contemporary cohort over a relatively short period where outcomes can reliably be recorded and verified 2. This is the first Australian study that demonstrated operative time as it relates to surgical technique, comparing pedicled and free flaps and also stratified by flap type, oncological resection, or comorbidity 3. Operations more than 9 hours duration were associated with an 89% increase in the odds of developing a surgical complication 4. Prolonged operative time was also significantly associated with length of stay and return to theatres 5. Future studies are needed to investigate associations with each component of operative time element separately in a multi institutional design