OBJECTIVES: To investigate whether there is a relationship between the day of the week patients undergo elective surgery and in-hospital death METHODS: Retrospective analysis of secondary users service data on adjusted odds ratios for in-hospital death within 30 days of procedure using logistic regression modelling. Patients in acute and specialist English public hospitals undergoing elective surgery over the financial years 2006 to 2008. RESULTS: There were 15,230 in-hospital 30-day deaths as a consequence of 4,197,761 all elective inpatient surgical admissions (an overall crude mortality rate 3.6 per 1000). The overall adjusted odds of death for all elective surgical procedures were 33% and respectively 90% higher if the procedures were carried out on Friday or at Weekend compared to Monday (OR 1.33, 95% CI 1.26-1.41 and respectively OR 1.90, 95% CI 1.75-2.05). This equates to a figure of over 700 excess annual deaths from elective surgery, based on applying Monday mortality rates CONCLUSIONS: The study suggests higher risk of death for patients who have elective surgical procedures carried out closer to the weekend, in particular on Friday. Further research is needed to understand the extent to which the ‘weekday effect’ reflects differences in the quality of surgical care.