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Background: Observationally, reports on the association between milk intake and risk of ischaemic heart disease (IHD) and myocardial infarction (MI) have produced conflicting results; and no previous large-scale study using the lactase persistent/non-persistent LCT-13910 C/T genotype as a largely unconfounded proxy for milk intake free of reverse causation has been conducted. We tested the hypothesis that milk intake observationally and genetically through the LCT-13910 C/T genotype is associated with risk of IHD and MI in a Mendelian randomization design. Methods: We included 98 529 White individuals of Danish descent, aged 20-100 years, from three studies of the general population. Information on IHD (N=10 372) and MI (N=4188) were obtained from national Danish registries. First, we investigated observational associations between milk intake and incident IHD and MI. Second, we confirmed the association between the rs4988235 genetic variant LCT-13910 C/T, associated with lactase persistence/non-persistence, and milk intake. Finally, we tested whether LCT-13910 C/T genotype was associated with risk of IHD and MI as well as with cardiovascular risk factors. Results: During a mean follow-up time of 5.4 years, the observational hazard ratio for a 1 glass/week higher milk intake was 1.00 [95% confidence interval (CI): 1.00,1.01] for both IHD and MI. Median milk intake was 3 glasses/week (interquartile range: 0-7) in lactase CC non-persistent individuals compared with 5 glasses/week (0-10) in lactase TC/TT persistent individuals (P=3*10-60). In the dominant genetic model comparing lactase TC/TT persistent individuals with lactase CC non-persistent individuals, the odds ratio was 1.00 (0.92,1.09) for IHD and 0.96 (0.84,1.09) for MI. Finally, in the dominant genetic model genotype was not associated convincingly with plasma levels of total cholesterol, low density lipoprotein cholesterol, high-density lipoprotein cholesterol, triglycerides or glucose, nor with blood pressure. Conclusion: Milk intake was not associated with risk of IHD or MI, observationally orgenetically [ABSTRACT FROM AUTHOR]