To estimate risk of parental cardiovascular disease mortality by offspring birthweight.Population-based cohort study.Norwegian mothers and fathers with singleton births during 1967-2002 were followed until 2009 by linkage to the Norwegian cause of death registry.Hazard ratios by offspring absolute birthweight in grams and birthweight adjusted for gestational age (z-score) were calculated using Cox regression and adjusted for parental age at delivery and year of first birth. Stratified analyses on preterm and term births were performed.Maternal and paternal cardiovascular mortality.We followed 711 726 mothers and 700 212 fathers and found a strong link between maternal cardiovascular mortality and offspring birthweight but only slight evidence of associations in fathers. Adjusting birthweight for gestational age (by z-score) uncovered an unexpected strong association of large birthweight (z-score > 2.5) with mothers' cardiovascular mortality (hazard ratio 3.0, 95% CI 2.0-4.6). This risk was apparently restricted to preterm births. In stratified analyses (preterm and term births) hazard ratios for maternal cardiovascular mortality were 1.5 (1.03-2.2) for large preterm babies and 0.9 (0.7-1.2) for large term babies (P-value for interaction = 0.02), using normal weight preterm and term, respectively, as references.Women having large preterm babies are at increased risk of both diabetes and cardiovascular mortality. The birth of a large preterm baby should increase clinical vigilance for onset of diabetes and other cardiovascular disease risk factors.Birth of a large preterm baby should increase vigilance for cardiovascular-disease risk factors.