INTRODUCTION Unplanned conversion to thoracotomy remains a major concern in video assisted thoracoscopic surgery (VATS) lobectomy. This study aimed to investigate the development of a VATS lobectomy programme over a five-year period, with a focus on the causes and consequences of unplanned conversions. METHODS A single centre retrospective review was performed of patients who underwent complete anatomical lung resection initiated by VATS between January 2010 and April 2015. RESULTS In total, 1,270 patients underwent a lobectomy in the study period and 684 (53.9%) of these were commenced thoracoscopically. There were 75 cases (10.9%) with unplanned conversion. The proportion of lobectomies started as VATS was significantly higher in the second half of the study period (2010-2012: 277/713 [38.8%], 2013-2015: 407/557 [73.1%], p<0.001). The conversion rate dropped initially from 20.4% (11/54) in 2010 to 9.9% (15/151) in 2013 and then remained consistently under 10% until 2015. Conversions were most commonly secondary to vascular injury (26/75, 34.7%). Patients undergoing unplanned conversion had a longer length of stay than VATS completed patients (9 vs 6 days, p<0.001). There was a higher incidence of respiratory failure (10/75 [14.1%] vs 23/607 [3.8%], p<0.001) and 30-day mortality (7/75 [9.3%] vs 6/607 [1.0%], p=0.003) in patients with unplanned conversion than in those with completed VATS. CONCLUSIONS As our VATS lobectomy programme developed, the unplanned conversion rate dropped initially and then remained constant at approximately 10%. With increasing unit experience, it is both safe and technically possible to complete the majority of lobectomy procedures thoracoscopically.