Background: This study examined whether behaviour problems and adaptive behaviour of low functioning young adults, and well-being of their families, varied by diagnostic syndrome intellectual disability (ID) only, cerebral palsy, Down syndrome, autism, as well as by cultural group. Methods: Behaviour disorders in young adults with moderate to severe ID were assessed from information provided by 282 caregivers during in-home interviews. The sample consisted of 150 Anglo participants, and 132 Latino, primarily Spanish-speaking, participants drawn from Southern California. Results: Behaviour disorders and maternal well-being showed the same pattern across disability syndromes. Autism was associated with the highest scores in multiple behaviour problem areas as well as maternal reports of lower well-being. Down syndrome was associated with the lowest behaviour problem scores and the highest maternal well-being. When behaviour problems were controlled for, diagnostic groups accounted for no additional variance in maternal stress or depression. The pattern of behaviour problems and well-being did not differ by sample (Anglo vs. Latino), although level on well-being measures did. Latina mothers reported significantly higher depression symptoms and lower morale, but also higher positive impact from their child than did Anglo mothers. Conclusions: Caregivers of young adults with autism report more maladaptive behaviour problems and lower personal well-being, or stress, relative to other diagnostic groups, regardless of cultural group. However, cultural differences exist in caregiver reports of depression, morale, and positive perceptions. Implications for service provision aimed at families of children with challenging behaviour problems are discussed in the context of culture.