Summary We compared the cost‐effectiveness of general anaesthetic agents in adult and paediatric day surgery populations. We randomly assigned 1063 adult and 322 paediatric elective patients to one of four (adult) or two (paediatric) anaesthesia groups. Total costs were calculated from individual patient resource use to 7 days post discharge. Incremental cost‐effectiveness ratios were expressed as cost per episode of postoperative nausea and vomiting (PONV) avoided. In adults, variable secondary care costs were higher for propofol induction and propofol maintenance (propofol/propofol; p < 0.01) than other groups and lower in propofol induction and isoflurane maintenance (propofol/isoflurane; p < 0.01). In both studies, predischarge PONV was higher if sevoflurane/sevoflurane (p < 0.01) was used compared with use of propofol for induction. In both studies, there was no difference in postdischarge outcomes at Day 7. Sevoflurane/sevoflurane was more costly with higher PONV rates in both studies. In adults, the cost per extra episode of PONV avoided was £296 (propofol/propofol vs. propofol/ sevoflurane) and £333 (propofol/sevoflurane vs. propofol/isoflurane).