OBJECTIVE: A randomised controlled trial (RCT) was designed to provide robust evidence about the relative costs, patient benefits and acceptability of alternative anaesthetic agents in day surgery. Staff costs are the principal component of semi-fixed costs for health-care interventions. There is no published information on the resource use associated with the staff involved during the day case episode. This study provided information on day-case staff deployment and skill mix to calculate associated resource use and semi-fixed costs. METHODS: Non-participant observation was used to measure staff resource use in the anaesthesia room and in the operating theatre for a sample of recruited patients. Face-to-face interviews were used to collect NHS staff’s subjective estimates of resource use associated with the remaining stages of the day-case episode. The NHS perspective was taken. Published estimates (price year 2000) of NHS staff salaries were used for unit costs. RESULTS: One hundred ninety four day-case procedures were observed in the anaesthetic room and operating theatre of the main study NHS Trust sites. Differences in working practices in terms of skill mix were observed between the three hospital sites. There was also a difference in the staff costs in the anaesthetic room and operating theatre for adult and paediatric day-case surgery: anaesthetic room—£0.91 per minute per adult patient (95% CI: £0.86 to 0.96, n = 157) and £1.42 per minute per paediatric patient (95% CI: £1.15 to 1.69, n=37, p<0.05); operating theatre—£2.15 per minute per adult patient (95% CI: £1.99 to 2.31, n=157) and £2.07 per minute per paediatric patient (95% CI £1.78 to 2.36, n=37). CONCLUSION: This study detected differences in the semi-fixed costs for adult and paediatric practice, and between sites. This suggests that differences in clinical practice between sites and patient groups must be accounted for in economic evaluations, or erroneous conclusions may be made.