Using the Head Start Impact Study data, this secondary data analysis examines Head Start's impact on cognitive outcomes for children in foster care. Out of 4442 children, 162 children in foster care were selected to examine the following study questions. (1) Do children in foster care who enrol in Head Start have different child and family characteristics than those who do not participate in Head Start? (2) Do children in foster care who participate in Head Start have higher reading and math scores at ages five to six? (3) Do child and family characteristics moderate Head Start's impact on reading and math scores of children in foster care at ages five to six? There was no main Head Start impact on reading and math scores for children in foster care. However, Head Start impact was found for the child's gender and the caregiver's age. Girls who participated in Head Start obtained higher reading and math scores than boys. Children cared for by older caregivers had higher math scores than those cared for by younger caregivers. Baseline variables such as ethnicity, special needs status and cognitive skills prior to Head Start enrolment were directly associated with math and reading scores at ages five to six. Copyright © 2015 John Wiley & Sons, Ltd. ‘Examines Head Start's impact on cognitive outcomes for children in foster care’ Key Practitioner Messages Child welfare workers should consider enrolling more children who are in foster care in the Head Start programme. Head Start should address and implement programme performance standards and regulation specifically targeting children in foster care. Head Start should consider various family characteristics to maximise programme benefits for children in foster care. ‘Child welfare workers should consider enrolling more children who are in foster care in the Head Start programme’