Our objectives were to assess the frequency of behavioural changes in patients with amyotrophic lateral sclerosis (ALS) and to compare the clinical profile of ALS patients with those with behavioural variant frontotemporal dementia (bvFTD). Ninety-two patients with ALS and their carers participated in a postal survey. ALS patients completed self-report measures of motor function and mood. Eighty-one carers of ALS patients and 25 carers of bvFTD patients completed the revised version of the Cambridge Behavioural Inventory (CBI-R). Results showed that reduced motivation was reported in more than 80% of the ALS cases, with almost 41% of them having moderate-severe apathy. Depression was present in 30% of ALS patients and did not contribute significantly to the presence of behavioural symptoms. Bulbar and limb onset ALS patients did not differ. Abnormal behaviour and stereotypical and motor behaviours were present to a moderate-severe degree in around 20%, and 11% reached the criteria for FTD. The rate of behavioural symptoms was significantly higher in the bvFTD group than ALS in all behavioural domains (p <0.001). In conclusion, apathy was the most prominent feature in ALS patients. A substantial proportion of ALS patients manifested behavioural changes of the type seen in FTD, with 11% fulfilling the criteria for FTD.