To date, the concept of regression, characterised by a loss of previously established skills somewhere in the second year of life, remains a puzzling phenomenon in the development of children with autism spectrum disorder (ASD). Based on increasing scientific and clinical interest, there is accumulating phenotypical and genetic evidence of a link between regression and restrictive and repetitive behaviours (RRBs). Prospective studies that are following-up the development of children with an older sibling with ASD who are at elevated likelihood for ASD (EL-sibs) are less impacted by a number of conceptual and methodological issues related to retrospective studies and can therefore provide a better understanding of both regression and RRBs. The aim of the present project is to investigate if children with ASD and regression display higher rates of RRBs, especially at later ages, indicating that deleterious effects of regression become more pronounced over time. To obtain representative results, the present study will include three large cohorts: EL-sibs, EL-group of preterm children, and children with a community-based diagnosis. To investigate this, data from cross-sectional and longitudinal studies will be analysed and an additional follow-up study will be conducted. The presence of a specific phenotype of ASD characterised by regression and higher rates of RRBs, could both impact aetiological knowledge as well as clinical guidelines on early detection and intervention.