OBJECTIVES Bispectral Index (BIS) monitoring systems monitor spontaneous electroencephalography, track sedative drug effects and help guide anaesthetic administration. Evidence shows such monitoring enables savings on anaesthesia and reduces adverse events compared with monitoring clinical signs only. However, the cost-effectiveness of monitors is uncertain. METHODS A cost-effectiveness model and budget impact analysis were developed to compare the outcomes of monitoring the depth of anaesthesia with BIS monitors compared with standard clinical monitoring. Over a five-year period, the model estimated the incremental cost per quality-adjusted life year (QALY) and incremental number of adverse events avoided in theatres with anaesthetists using BIS monitors compared with standard care. Data values were obtained from peer-reviewed literature. Subgroup analysis was conducted for an elderly patient group. Sensitivity analyses explored uncertainty in the model. A budget impact model examined the financial impact of adopting BIS monitors across the UK. RESULTS The modelled results showed that using BIS monitors dominated clinical observation of signs and use of electrocardiograph and other devices, being cost saving by £82 per operation in the adult population, whilst improving QALYs by 0.016 per patient. The budget impact analysis showed a cumulative saving of over £136 million if theatres in the UK adopted a phased increase in monitor use such that 1.35 million surgical procedures were conducted using  these monitors in five years time. CONCLUSIONS Adopting BIS monitors is cost-effective and results in substantial cost savings compared with observing clinical signs plus conventional devices only.