Background: The hallmark of attachment is that contact, proximity and relief from stress are sought from specific individuals, laying important groundwork for healthy socioemotional functioning. This study investigated the extent to which differentiated attachment behaviour can be observed in young children with significant developmental delay (DD). Method: Video-taped observations of the parent-child and stranger-child interaction were conducted at home and complemented with questionnaires in 20 families with a child with significant DD (age 2-7 years with an average DD of 49 months). Results: Children displayed more intense and persistent contact-seeking, contact-maintaining and resistant behaviour in the episodes with their parent compared to the episodes with the stranger. Parent-reported secure attachment behaviour was slightly more characteristic towards mother compared to father. Conclusions: Even children with significant DD develop differentiated attachment behaviour. Detailed observations may support parents in identifying the interactions that make the attachment relationship with their child special.