Background: Frontotemporal dementia (FTD) presents with marked neuropsychiatric symptoms that impacts negatively on activities of daily living (ADLs). The relationship between neuropsychiatric symptoms and disability in FTD is not well understood. This study aimed to (1) describe baseline function and neuropsychiatric symptoms in behavioural variant FTD (bvFTD) and semantic variant primary progressive aphasia (svPPA); (2) examine longitudinal functional and neuropsychiatric symptom changes; and (3) examine whether function and neuropsychiatric symptoms are associated longitudinally. Methods: 39 people (bvFTD=21; svPPA=18) were seen annually on two to four visits between 2007-2012, with 200 assessments completed in total. The Disability Assessment for Dementia (DAD) assessed function, Cambridge Behavioural Inventory-Revised (CBI-R) evaluated neuropsychiatric symptoms , and Addenbrooke’s Cognitive Examination Revised (ACE-R) measured general cognition. Results: At baseline, people with bvFTD were more impaired functionally than svPPA despite having higher cognitive scores. In terms of baseline neuropsychiatric symptoms, bvFTD showed higher levels of apathy and changes to eating habits, whereas there was no difference in disinhibited or stereotypical behaviours between the groups. Longitudinally, both groups showed functional decline, however bvFTD remained worse than svPPA over time. Apathy and eating changes increased for both groups. Over time, disinhibited and stereotypical behaviours reduced in bvFTD, yet continued to increase for svPPA. Apathy and stereotypical behaviour were associated with functional decline in bvFTD, whereas only apathy made longitudinal contributions to functional disability in svPPA. Conclusions: People with bvFTD are more functionally impaired than svPPA, and despite overlapping baseline neuropsychiatric symptoms, these dissociate over time, with disinhibition and stereotypical behaviour reducing in people with bvFTD and increasing in svPPA. For both bvFTD and svPPA, apathy has a strong role in disability, but stereotypical behaviors only contributed to functional deficits in bvFTD. Aligning with previous clinical observations, our findings suggest that rigid/compulsive behaviors may in fact support activity engagement in svPPA (O'Connor et al. Disability & Rehabilitation 2016). Overall, our results indicate that interventions to address functional disability in FTD should carefully weigh the interaction of neuropsychiatric symptoms and functional status, thus require a tailored approach which may differ from those used in AD dementia.