IntroductionAmyotrophic lateral sclerosis (ALS) is an incurable motor neuron degenerative disease that has rapid progression and is associated with cognitive impairment. For people with ALS (pALS) and their family carers, advance care planning (ACP) is beneficial, as it can lead to feelings of control/relief and refusal of unwanted treatments. However, evidence concerning the experiences and preferences regarding ACP of pALS and their family carers, especially when their symptoms progress, is scarce. This article describes the protocol for a qualitative longitudinal study that aims to explore: (1) the experiences with ACP and the preferences for future care and treatment of pALS and their family carers and (2) how these experiences and preferences change over time.Methods and analysisA qualitative, longitudinal, multiperspective design. A total of eight to nine dyads (pALS and their family carers) will be recruited, and semistructured interviews administered every 3 months over a 9-month period. Qualitative longitudinal analysis involves content analysis via in-depth reading, followed by a two-step timeline method to describe changes in experiences and preferences within and across participants.Ethics and disseminationThis protocol has been approved by the central ethical committee of the University Hospital of Brussels, and local ethical committees of the other participating hospitals (B.U.N. B1432020000128). The results will be disseminated via the research group’s (endoflifecare.be) website, social media and newsletter and via presentations at national and international scientific conferences.