Effective decision-making has the potential to facilitate improvements in health care. This paper reports several aspects of a study which used "thinking aloud" within a concept attainment framework to examine the decision-making processes of expert critical care nurses in relation to haemodynamic monitoring. The purpose of this study was to examine whether hypotheses were used in the decision-making process and, if so, were hypotheses deactivated when no longer relevant. In addition, the strategies that were used during the decision-making process were examined. Eight expert critical care nurses consented to participate in the study after ethics clearance was obtained from both the University and Hospital ethics committees. The majority of expert critical care nurse participants in this study demonstrated extensive use of hypotheses to explain the relationship between attributes and concepts. There was no evidence of specific deactivation of hypotheses when they were no longer relevant. Participants demonstrated use of a range of decision-making strategies, with a Focus Gambling Strategy being the most common. The reason for using different decision-making strategies was not clear and may represent variation between decision-makers or between scenarios for each individual decision-maker. Recommendations are made to help improve critical care nurses' decision-making.