The aim of the study was to assess Oxford Miniature Vaporizer output when mounted in-circuit during the maintenance phase of anaesthesia, using isoflurane, controlled ventilation and a fresh gas flow rate less than 1 l/min. Twenty patients of ASA Physical Status I and II were recruited from routine general surgical lists. All patients were paralysed and ventilated. An out-of-circuit isoflurane vaporiser was used during the induction period (first 20 to 30 minutes). Anaesthesia was maintained using an Oxford Miniature Vaporizer placed in-circuit, using a fresh gas flow of 500 ml/min. The end-tidal isoflurane concentration was recorded for 90 minutes at five-minute intervals using a sidestream agent analyser. Two groups were compared, with the Oxford Miniature Vaporizer dial setting at either the 0.5 mark (low output setting) or at the 1.0 mark (higher output setting).At a dial setting of 0.5, the Oxford Miniature Vaporizer produced a steady end-tidal isoflurane of 0.63% (95% confidence interval 0.60 to 0.66). However, when the dial was turned to 1.0 the output was almost always excessive and had to be reduced.These findings indicate that a stable, predictable and clinically useful output can be achieved when the Oxford Miniature Vaporizer is positioned in-circuit using low-flow and controlled ventilation.