Evaluation of interventions for children with autism spectrum disorder (ASD) is hampered by the multitude of outcomes measured and tools used. Measurement in research with young children tends to focus on core impairments in ASD. We conducted a systematic review of qualitative studies of what matters to parents. Parent advisory groups completed structured activities to explore their perceptions of the relative importance of a wide range of outcome constructs. Their highest ranked outcomes impacted directly on everyday life and functioning (anxiety, distress, hypersensitivity, sleep problems, happiness, relationships with brothers and sisters, and parent stress). Collaboration between professionals, researchers and parents/carers is required to determine an agreed core set of outcomes to use across evaluation research.