REM sleep behaviour disorder (RBD) is a neurological condition well known to be associated with the synucleinopathies in middle-aged patients. However, there is much less data concerning its development, evolution, and association with other disorders in younger patients. We report two patients aged less than 33 years who presented with clinical and polysomnographical features of RBD, both of whom proved to have previously undiagnosed narcolepsy. Whilst the association of narcolepsy with RBD has been previously recognised, this is the first report of narcoleptic patients presenting with RBD. Narcolepsy should be included in the differential diagnosis of young patients presenting with abnormal behaviour during sleep compatible with RBD.