End-tidal anaesthetic gas concentration monitoring during inhalation anaesthesia has become standard of care. Explorative study comparing the end-tidal anaesthetic gas measured by a new main-stream anaesthetic gas monitor (IRMA, Phasein, Stockholm, Sweden) as compared to standard Datex side-stream monitoring during routine low flow anaesthesia Day surgical centre in StockholmThirty two healthy, ASA 1-2, patients undergoing elective day case anaesthesia with low flow 0.2-0.5 L/min. Simultaneous recording of the end-tidal gas concentration during routine day case anaesthesia with low flow sevoflurane or desflurane anaesthesia. The new monitor was found to be clinically acceptable. The mean bias between IRMA and Datex measures -0.125 vol % (± 0.145, limits of agreement were -0.41 – 0.16). The new main stream gas monitor is clinically acceptable alternative for end-tidal anaesthetic gas monitoring during routine anaesthesia with low flow, 0.2-0.5 L/min.