Background: The purpose of this study was to examine the correlates of caregiver stress in a large sample of young people with autism spectrum disorders (ASDs). Two main objectives were to: (1) disentangle the effects of behaviour problems and level of functioning on caregiver stress; and (2) measure the stability of behaviour problems and caregiver stress. Methods: Parents or teachers of 293 young people with ASDs completed measures of stress, behaviour problems and social competence. Parents also completed an adaptive behaviour scale. Eighty-one young people were rated twice at a 1-year interval. Results: Parents and teachers did not perfectly agree on the nature and severity of behaviour problems. However, both sets of ratings indicated that behaviour problems were strongly associated with stress. Conduct problems in particular were significant predictors of stress. Adaptive skills were not significantly associated with caregiver stress. Parental reports of behaviour problems and stress were quite stable over the 1-year interval, much more so than teacher reports. Parent ratings suggested that behaviour problems and stress exacerbated each other over time. This transactional model did not fit the teacher data. Conclusion: Results of this study suggested that it is a specific group of externalized behaviours that are the most strongly associated with both parent and teacher stress. Results were discussed from methodological and conceptual perspectives.