We analysed more than 7000 theatre lists from two similar UK hospitals, to assess whether start times and finish times were correlated. We also analysed gap times (the time between patients when no anaesthesia or surgery occurs), to see whether these affected theatre efficiency. Operating list start and finish times were poorly correlated at both hospitals (r 2 = 0.077 and 0.043), and cancellation rates did not increase with late starts (remaining within 2% and 10%respectively at the two hospitals). Start time did not predict finish time (receiver operating curve areas 0.517 and 0.558, respectively), and did not influence theatre efficiency ($80-84% at either hospital). Median gap times constituted just 7% of scheduled list time and did not influence theatre efficiency below cumulative gap times of less than 15% scheduled list time. Lists with no gaps still exhibited extremely variable finish times and efficiency. We conclude that resources expended in trying to achieve prompt start times in isolation, or in reducing gap times to under $15% of scheduled list time, will not improve theatre productivity. Instead, the primary focus should be towards quantitative improvements in list scheduling. ('knife-to-skin'), because if one of these is fixed then the other follows automatically as a probabilistic estimate [6]. All that matters is that a clear, pre-agreed definition exists within an institution. Prompt starts would seem important. If a list starts late, it should be expected to finish late. With late starts, the scheduled time (which has been budgeted in advance) will be wasted and with late finishes, unbudgeted overtime costs are incurred, with the attendant risk of unplanned patient cancellation. Several studies, however, have found a very high prevalence of non- Anaesthesia 2012Anaesthesia , 67, 823-832 doi:10.1111Anaesthesia /j.1365Anaesthesia -2044Anaesthesia .2012 Consequently, start times are monitored routinely within many NHS Trusts, and are used both as local 'key performance indicators' to guide planning, and, occasionally, to pressurise individuals or teams to change behaviour, attitudes or policies [11,12]. The Foundation Trust Network has promoted start times as a key target [13] and the Royal College of Anaesthetists has suggested that fewer than 10% of lists should start more than 10 min late [14]. However, whilst the published evidence has found that late starts are frequent, it does not indicate that late starts affect theatre efficiency. Furthermore, there is no evidence to substantiate the Royal College's suggestion that 10 min should be a relevant time boundary when defining a 'late start'. Indeed, Macario has suggested (but without citing data to support the claim) that late starts of up to 45 min remain consistent with efficient performance [15,16].The main aim of this study was to test the hypothesis that start time was an important determinant of operating list efficiency. Specifically, we predicted that lists that started late would end late and be less efficient.As late starts rep...