Objective: The objective of this study was to examine how specialists should provide support for amyotrophic lateral sclerosis (ALS) patients in the process of their decision-making regarding the use of mechanical ventilation. A case analysis was made focused on the changes in a patient's perception through the process of her interaction with her family members and specialists. Methods: The author visited the patient's home regularly, and conducted interviews with the patient, her family and the specialists involved. Results: There were perceptual differences regarding the use of mechanical ventilation among the patient, her family and specialists. These differences arose because of the differences in the patient's perspective, which focused on the present, and that of the specialists, which focused on future developments. Problems resulting from information gaps were resolved by ongoing professional involvement including the provision of information, while problems arising from different values were difficult to deal with. Conclusion: Specialists should be aware of these differences when offering support. Besides providing accurate information, they should facilitate interaction between the patients and family members, thus helping them to reach a consensus.