Blood samples and questionnaires were obtained from about 25,000 MI patients entered in the ISIS trials, and from about 25,000 age- and sex-matched controls. As a result of its large size, this case-control study can assess more reliably than before the relative importance of various known and suggested risk factors for MI.

For example, it showed that smoking was an even more important cause of heart attacks at younger ages than had been thought to be the case. Other factors now being assessed include: alcohol consumption; lipid fractions and apolipoproteins; antibodies to helicobacter pylori, chlamydia pneumoniae and other infective agents; fibrinogen and other haemostatic factors.

DNA has been extracted by the Oxford Wellcome Trust Centre for Human Genetics (WTCHG) from the samples of over 12,000 younger cases and controls. This is by far the largest genetic case-control comparison of any disease that has been conducted, and provides an opportunity to test reliably various hypotheses generated by much smaller studies. For example, it has already refuted the prominently published claims that the angiotensin converting enzyme insertion/deletion polymorphism is importantly related to MI, and has now generated fascinating new information about certain fibrinogen- and lipid-related genes.