Twins constitute a population with lower than average birth weight for reasons that are not a consequence of social disadvantage. The hypothesis that ischaemic heart disease (IHD) is linked to low birth weight was tested by analysing whether or not 8174 female and 6612 male Swedish twins had a higher mortality compared to the general Swedish population. The association between adult body height and IHD mortality was also analysed in a nested case-control study among monozygotic and dizygotic twins. Ischaemic heart disease mortality was not higher among twins (women: relative risk [RR] 0.99; 95% confidence limits [CL] 0.89-1.10; men: RR 0.85; CL 0.79-0.92). However, the shorter twin in a twin pair was more likely to die of heart disease than the taller (odds ratio [OR] 1.15, CL 1.03-1.25). We suggest that postnatal influences may well be as important as prenatal influences in producing any effect on ischaemic heart disease mortality and that the type of growth retardation in utero experienced by twins may not constitute a risk for ischaemic heart disease in adulthood.