Pregnancy complications and cardiovascular disease share some common determinants. It has previously been hypothesized that family history of cardiovascular disease would be associated with low birth weight. Records from 120,317 Scottish births, 1992-2006, were linked to hospital admission and death certificate data for 71,681 pairs of maternal grandparents. There was a negative relation between the birth weight of the baby and the risk of either grandparent's experiencing ischemic heart disease (for a 1-kg increase in birth weight, hazard ratio = 0.86, 95% confidence interval: 0.83, 0.89) or cerebrovascular disease (hazard ratio = 0.82, 95% confidence interval: 0.77, 0.87). Further analysis demonstrated that the associations were explained by increased risks of both delivering a small-for-gestational-age infant and delivering preterm among women whose parents had experienced cardiovascular disease. Adjustment for the mother's characteristics at the time of the birth attenuated the relation, but significant associations persisted: With a 1-kg increase in birth weight, the adjusted hazard ratio for ischemic heart disease = 0.93 (95% confidence interval: 0.89, 0.96) and for cerebrovascular disease = 0.93 (95% confidence interval: 0.89, 0.96). Familial aggregation of common determinants of pregnancy complications and cardiovascular disease is the likely explanation for the relation between an infant's birth weight and the risk of cardiovascular disease in other family members.