Aims: To investigate the association of breastfeeding with all-cause, cardiovascular, and ischaemic heart disease mortality. Methods and results: A long-term follow-up of 4999 children originally surveyed from 1937 to 1939 was undertaken (Boyd Orr cohort). Four thousand three hundred and seventy-nine subjects (88%) were traced in adulthood and 3555 (71%) had complete data on all covariates. The results were combined with a meta-analysis of the published literature. In the Boyd Orr study, there was little evidence that breastfeeding was associated with all-cause (hazard ratio: 1.04 [95% CI: 0.90-1.20]), cardiovascular (1.04 [0.83-1.30]), or ischaemic heart disease (1.02 [0.77-1.36]) mortality, compared with bottle-feeding. Meta-analyses of observational studies showed little evidence of an association of breastfeeding with all-cause (pooled rate ratio: 1.01 [95% CI: 0.91-1.13]) or cardiovascular (1.06 [0.94-1.20]) mortality. There was a moderate-to-high degree of between-study heterogeneity for the association between breastfeeding and ischaemic heart disease mortality (I2 value-indicating the degree of between-study variation attributable to heterogeneity-66%), and estimates were consistent with both an important beneficial or adverse effect of breastfeeding.