Key PointsQuestionIs a family history of sudden infant death syndrome (SIDS) associated with increased risk of subsequent SIDS compared with the general population?FindingsIn this cohort study of more than 2.6 million consecutive births in Denmark between 1978 and 2016, a total of 1540 infants died of SIDS. A higher rate of SIDS was observed among siblings of children who died of SIDS compared with the general population.MeaningThese findings suggest that any sibling of a child who died of SIDS should be investigated with great care to exclude genetic and environmental factors.