We wished to determine whether patients with amyotrophic lateral sclerosis (ALS) show behavioural changes similar to those of frontotemporal dementia (FTD). There is accumulating evidence to suggest a link between ALS and FTD, yet there has been little systematic exploration of behaviour in ALS. An informant based semi-structured behavioural interview, sensitive to the behavioural changes of FTD, was administered to carers of 16 consecutive patients attending a motor neuron disease clinic. Findings varied across the group. At one extreme informants reported no behavioural change, whereas at the other they reported a spectrum of behaviours similar to those seen in FTD. Changes in affect and social behaviour were most common, although some patients also showed altered response to sensory stimuli, gluttony and indiscriminate eating, behavioural stereotypies and compulsions. Behavioural changes were mirrored by SPECT abnormalities in the frontal and/or temporal lobes. Thus, behavioural changes of the type seen in FTD may be present even in a small consecutive cohort of ALS patients. Detection of behavioural change is crucial for optimal management.